Reproduction and dispersal of goshawks in a variable environment

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Reproduction and dispersal of goshawks in a variable environment

Patrik Byholm

Bird Ecology Unit

Department of Ecology and Systematics Division of Population Biology

P.O. Box 65 (Viikinkaari 1) FIN-00014 University of Helsinki

Finland

Academic dissertation

To be presented with the permission of the Faculty of Science of the University of Helsinki, for public criticism in the Auditorium 2 of Viikki Info Centre (Viikinkaari 11) on April 25^th2003 at 12 o’clock noon

Helsinki 2003

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Cover illustration: Kenneth Rosenlund “At the goshawk’s nest”

Author’s address:

Bird Ecology Unit

Department of Ecology and Systematics Division of Population Biology

P.O. Box 65 (Viikinkaari 1) FIN-00014 University of Helsinki Finland

e-mail: patrik.byholm@helsinki.fi ISBN 952-91-5552-2 (paperback) ISBN 952-10-0951-9 (PDF) http://ethesis.helsinki.fi Edita Prima Oy Helsinki 2003

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Reproduction and dispersal of goshawks in a variable environment

Patrik Byholm

The thesis is based on the following articles, which are referred to in the text by their Roman numerals:

I Byholm, P., Brommer, J. E. and Saurola, P. 2002: Scale and seasonal sex-ratio trends in northern goshawkAccipiter gentilisbroods. – Journal of Avian Biology 33: 399–406.

II Byholm, P., Ranta, E., Kaitala, V., Lindén, H., Saurola, P. and Wikman, M.

2002: Resource availability and goshawk offspring sex ratio variation:

a large-scale ecological phenomenon. – Journal of Animal Ecology 71:

994–1001.

III Ranta, E., Byholm, P., Kaitala, V., Saurola, P. and Lindén, H. 2003:

Spatial dynamics in breeding performance of a predator: the connection to prey availability. – Oikos, in press.

IV Byholm, P. 2003: Partial brood-loss and offspring sex ratio in goshawks.

– Submitted.

V Byholm, P., Saurola, P., Lindén, H. and Wikman, M. 2003: Causes of dispersal in Northern Goshawks (Accipiter gentilis) banded in Finland.

– Auk 120, in press.

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4 Contributions

Contributions

The following table shows the major contributions of authors to the original articles or manuscripts.

I II III IV V

Original idea PB PB ER, PB PB PB

Materials PB, PS PB, HL, PS, MW PB, HL, PS, MW PB PB, HL, PS, MW

Analyses PB PB, ER ER, PB PB PB

Manuscript preparation PB, JB PB, ER, VK PB, ER, VK PB PB JB: Jon Brommer, PB: Patrik Byholm, VK: Veijo Kaitala, HL: Harto Lindén, ER: Esa Ranta, PS: Pertti Saurola, MW: Marcus Wikman. In addition several persons assisted with a variety of practical tasks. Their contributions are acknowledged in the relevant parts of the thesis.

Supervised by Prof. Hannu Pietiäinen University of Helsinki Finland

Reviewed by Doc. Hanna Kokko University of Jyväskylä Finland

and

Prof. Mikko Mönkkönen University of Oulu Finland

Examined by Prof. William J. Sutherland University of East Anglia The United Kingdom

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Summary

Introduction

While some decades ago most ecologists saw the world as a homogeneous en- tity inhabited by continuous populations, few would agree with this view today. Instead, there is wide agreement that large populations are usually split into sub-units, inhabiting a diverse landscape that has varying effects on the population renewal processes (Tilman and Karieva 1997). Of course, these sub-units are not isolated from each other, but are in connection by dispersal (Clobert et al. 2001). Births and deaths in local units are linked through immi- gration and emigration. Dispersal and reproductive success are thus the basal elements contributing to population fluctuations.

When analysing empirical data in larger spatial-temporal settings, population dynamical patterns themselves have become to dominate the scenery (e.g., Ranta et al. 1995b, Ranta et al. 1997a, Kendall et al. 1999, Swanson and Johnson 1999, Lundberg et al. 2000, Lindström et al. 2001, Stenseth et al.

2002), whereas the basic elements actually shaping the population fluctuation patterns, i.e. reproductive success and dispersal, have received less attention.

The reasons for this bias might be manifold, but at least in studies on reproductive success, it is certainly partly the outcome of a tradition where the nor- mal agenda is to analyse patterns from a single location only over a certain period of time (Clutton-Brock 1988, Newton 1989, Newton 1998). There might not be anything to lose because of this, as ultimately of course it is the single individual that responds to the surrounding local environmental conditions when ’deciding’ how many young to produce or whether to disperse or not (Clutton-Brock 1988, Clobert et al. 2001). If environmental conditions differ between sites and individuals respond to these conditions, however, this variability will generate spatially variable patterns of reproductive success and/or dispersal. In such situations there is a need to compare information from multiple sites in order to draw the correct conclusions concerning the generality of the results. Similarly, the spatial dimension must be considered important when addressing questions of whether different facets of reproduction and dispersal are mediated by the same factor(s) over the same or different ecological scales. Regarding many demographic elements/measurements this is not well known. For example, is the decision to disperse determined by the same factor(s) as the decision how long to disperse and at which scales do these decisions take place?

Sex allocation in vertebrates which have chromosomal sex determination is one scientific domain where the summed information gained from multiple sites (as compared to information acquired from a single location) has great potential to add to our understanding of how an ecological process actually works. Only some ten years ago there was little evidence for substantial sex ratio variation among offspring in any avian and mammalian species (Wil- liams 1979, Clutton-Brock 1986, Clutton-Brock and Iason 1986, Breitwisch

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1989). It was widely accepted that the observed 1:1 sex ratio normally present at the population level was the result of the Mendelian process of meiosis (Williams 1979), or alternatively, frequency-dependent selection leading to equilibrium simply because overproduction of the one sex would lead to selection in favour of the under-endowed sex because of its greater fitness ad- vantage in that situation (Fisher 1930). While these theories are aimed at the population level, individual-based theory implies that it would be unexpected if deviation from the 1:1 sex ratio werenotto be found if fitness returns from producing one sex differ from those of producing the other sex, in parallel with some covariate in the individuals’ ecological or social environment (Trivers and Willard 1973, Charnov 1982, Frank 1990, Wright et al. 1995, Daan et al. 1996, Leimar 1996, Pen et al. 1999). For example, if the fitness in- come from producing sons is higher than from producing daughters during periods when prey is abundant, pairs living in good food regimes should have a male-biased sex ratio (sensuTrivers and Willard 1973).

The introduction of DNA-based tools designed to determine the sex of (nestling) birds in the 1990s (Ellegren and Sheldon 1997, Griffiths et al. 1998) has revolutionised sex-allocation studies in birds, because many species could not be reliably sexed earlier due to sexual monomorphism in morphological characters. Consequently, the amount of studies testing the predictions of individual-level sex allocation theory has rapidly increased. More in- terestingly, some studies have convincingly demonstrated that bird species from several systematically distant families are somehow capable of manipu- lating the sex of their eggs in line with the theoretical predictions (reviewed in Godfray and Werren 1996, Sheldon 1998, Bensch 1999), even though the exact mechanism by which this is achieved is so far unknown (Cockburn et al.

2002). It is important to realise that not all sex-ratio patterns described in the literature are necessarily the result of facultative manipulation of offspring sex at the moment of conception.

Especially in birds, which as a group are well known to experience secondary family size adjustments after hatching (Lack 1954, Mock and Parker 1997), it is also possible that sex-biased mortality can be the main force gener- ating the sex ratio observed among nestlings (Howe 1977, Fiala 1980, Dijkstra et al. 1998, Hipkiss et al. 2002). Naturally, facultative manipulation of egg sex and sex-linked mortality can also interact (e.g. Nager et al. 2000).

However, irrespective of whether the patterns observed are the result of facultative manipulation, sex-biased mortality or both, empirical work has reported highly varying sex-ratio patterns between different populations and/or seasons within the same species (e.g. Edwards et al. 1988, Wiebe and Bortolotti 1992, Lessells et al. 1996, Svensson and Nilsson 1996, Kölliker et al. 1999, Sheldon et al. 1999, Korpimäki et al. 2000). This inconsistency has raised doubt over whether the sex ratio patterns observed among birds, as well as mammals, are real or just statistical noise resulting from stochastic variation in small sample sizes (Palmer 2000, Brown and Silk 2002). Clearly, there is thus a need for replicate studies (comparisons of several populations) before far-going conclusions about sex-ratio variation and its potential adaptive significance can be drawn.

The aims of this thesis were to characterise and investigate spatial and temporal patterns of reproductive success and dispersal using the northern goshawksAccipiter gentilis(L.) (from here on goshawk) as a model species,

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particularly in relation to local densities of their main prey, forest grouse Tetraonidae. Special emphasis was put on characterising and describing patterns of sex-ratio variation among goshawk offspring. Bearing in mind the possibility that the sex ratio among young produced by individual mothers is not necessarily random (either determined facultatively at conception, by sex-biased mortality, or both), the option that such non-random allocation also might come to influence offspring sex ratios in whole local populations was analysed. This would be the case if, for example, sex ratios were strongly associated with local environmental quality and individuals manipulated the sex of their offspring in line with the quality of the environment. The same idea was tested also for dispersal and variation in the number of breeding goshawk pairs, clutch size, brood size and partial brood-loss. The work was con- ducted on variable spatial and temporal scales, and using the combined results from multiple sites, the idea was to test the generality of the patterns observed and their association to relevant parts of theory, especially sex ratio theory (Trivers and Willard 1973, Charnov 1982, Frank 1990).

The goshawk

The goshawk is a long-lived, medium-sized and size-dimorphic raptor, with high mate and territory-fidelity. It is widely distributed in the Nearctic and Palaearctic regions (Cramp and Simmons 1980, Squires and Reynolds 1997, Ferguson-Lees and Christie 2001). Throughout the range, medium-sized birds and mammals are the most important prey items, forest grouse being the most important prey group in Finland, both in summer and in winter (Sulkava 1964, Lindén and Wikman 1983, Tornberg 1997, Tornberg and Colpaert 2001). Because of its smaller size, the male hunts somewhat smaller prey than the female (Cramp and Simmons 1980, Tornberg 2000). As in most parts of their distribution Finnish goshawks mainly breed in large forest areas dominated by conifers. The large stick-nest, which is commonly used for several years (occasionally even over several decades) is usually placed in the middle parts of a Norwegian sprucePicea abies(L.), more seldom in Scots pines Pinus sylvestrisL. or in deciduous trees. In Fennoscandia, the breeding season starts with egg-laying in early- or mid-April (Tornberg 1997), and in common with most other species of birds of prey, the larger female is responsible for most of the incubation as well as for most of the rest of the parental care (brooding, guarding, feeding young). The smaller male has the main respon- sibility in providing the incubating female and chicks with prey during the whole breeding season (Cramp and Simmons 1980). After a parental-care period of three and a half to four months, the young reach independence in August–September after which they disperse (Kenward et al. 1993a, 1993b).

Material

The material I have used in this thesis originates from five different sources.

First, nationwide material on goshawk brood sizes and offspring sex ratios

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during 1989–1998 (n= 5,455) was extracted from the ringing-data files at the Finnish Ringing Centre, Museum of Natural History in Helsinki. Only material from broods with known size and sex-composition (reported by individual ringers) was used; as the goshawk is a species with pronounced sexual size-dimorphism (females 60% larger than males) the sex of goshawk nestlings can be accurately determined by morphological means. A double-check of the reliability of sexing done by morphological means, using DNA-sexing as an independent method, revealed no sexing-errors among 157 nestlings sexed by nine independent ringers (Patrik Byholm and Jodie Painter, unpubl.). This material was used in papersI,IIandIIIin order to describe patterns of spatial variation in goshawk breeding success (brood size) and offspring sex ratio as compared between locations all over Finland, as well as to test the regularity in these patterns in relation to various covariates (e.g., timing of breeding, prey availability).

Second, as grouse are important food for Finnish goshawks, nationwide material on local grouse densities (individuals/km²) during 1989–2002 were used as a measure of environmental quality in papersII,III,IVandVin relation to relevant questions of goshawk breeding and dispersal. The grouse material (concerning four species: black grouseTetrao tetrix(L.), willow grouse Lagopus lagopus (L.), hazel grouseBonasa bonasia (L.) and capercaillie Tetrao urogallusL.) was obtained from the Finnish Game and Fisheries Re- search Institute (FGFRI) from data collected under the Wildlife Triangle Programme (Lindén et al. 1996).

Third, nationwide information on annual goshawk breeding numbers (number of nests and occupied territories,n= 17,837) during 1986–2001 was extracted from the so-called“Raptor Questionnaires”data base at the Finn- ish Ringing Centre. This is a massive source of data, the goshawk material being a part of it, containing information on occupied raptor nests and territories with over 40,000 potential nest sites being checked annually (Saurola 1997, Hannula et al. 2002). This material was used in studyIIIin order to analyse the patterns of spatial synchrony in the number of breeding goshawk pairs.

Fourth, detailed material on goshawk reproductive success was collected in the field during 1999–2002, east from the small town of Närpes (62°28’N, 21°20’E) at the Finnish west coast in Pohjanmaa, in an area roughly spanning 6300 km², the main study area (see Fig. 1). Relevant parts of this material are presented in paperIV, which describes the patterns of variation in clutch size and partial brood-loss (with a special emphasis on whether nestling mortality is sex-biased or not) between different sites (territories, sub-regions) and years in 123 different territories, also taking into account variation in natural food (grouse) availability.

Fifth, data on goshawk dispersal, as determined from selected ring-recoveries in 1989–2000 (n= 272) was analysed in paperV. As factors affecting the dispersal behaviour of juveniles may differ from those that govern the dispersal behaviour of adults, especially in long-lived species (Kenward et al.

2001), dispersal of juvenile goshawks during their first winter (October 1–

February 28) and adults (+3 y) goshawks was analysed separately. All dispersal-data was obtained from the Finnish Ringing Centre. As both the ringing and the recovery coordinates were known to at least the nearest kilometre, the dispersal distance of all birds used in the analyses was known. Whether or not a bird dispersed (0/1–response) from a natal area (50 × 50 km grid) was

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also considered. Both concerning dispersal distance and the decision to stay or to go, the objective was to clarify what are the ecological and demo- graphical factors experienced by individual dispersers adding to these decisions.

Results and discussion

Spatial and temporal variation in goshawk breeding parameters

Variation in brood size, timing of breeding and number of breeding pairs

As in most other large-sized Accipitridae (Ferguson-Lees and Christie 2001), goshawk brood size varies within a quite narrow range. The most common broods consisted of two–four fledglings (90%), while broods of one (9%) or five (1%) were less common (I, Fig. 5b). However, brood size varied significantly temporally, both as analysed within and between years (I,II). In line with patterns observed in many other bird species, early goshawk broods were larger than late broods and brood size declined significantly with ad- vancing season. It is, however, evident that the steepness of the laying date-

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Norway

Sweden

Russia

H meenlinnaä Närpes

60° 30°

100 km

Kauhajoki Kristine-

stad Kaskö

Närpes

Fig. 1.Map of Finland with 50 × 50 km grids (n= 115, in grey) in which ringed$1 goshawk nestling in 1989 – 2000 (II,III,V). The city of Hämeen- linna in the interior of southern Finland (I) and the main study area (Poh- janmaa,I,IV) at the west coast east from the small town of Närpes are indi- cated. The exact border of the main study area is indicated by the dotted line on top of municipality borders in the left-side panel of the figure. Roman letters refer to papersI–V.

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brood size slopes can vary between geographically rather nearby regions (I).

Even though the pattern was highly variable spatially, inter-annual variation in average goshawk brood size in general coincided with annual variation in local grouse density, the main prey of Finnish goshawks. In a majority of a set of 50 × 50 km grid units (n= 73, see Fig. 1), the correlation between local grouse density and goshawk brood size was positive, i.e., the more grouse available the larger was the goshawk brood size (II). In addition, there was also clear large-scale spatial synchrony in local average goshawk brood size within Finland and the synchrony only modestly fell off with increasing distance. Thus, as highlighted in Figs. 2a and 3a, goshawk brood size correlates well even between distant areas (I,III). While the pattern of synchrony in lay- ing dates has not been tested more rigorously yet, available results suggests that also the timing of breeding in different parts of Finland is synchronised over large scales (Fig. 2b, cf.I). This is what would be expected as laying date to a various degree determines the brood size (I), which fluctuates in synchrony over wide areas (III). Concerning breeding pairs (Fig. 3b), the degree of synchrony resembles that of brood size and laying date, and in fact the spatial synchrony was at its maximum in this data-set. Consequently, when there were many goshawks breeding in southern Finland, the number of pairs breeding locally was also high 1,000 km further north in Lapland. At the same time, the population fluctuations of the goshawks’ main prey, grouse, were synchronous over large areas too, but here the synchrony, both in juveniles and in adult birds, rapidly died away when distance increased (III). This can be seen as differing grouse densities between different parts of the country (II).

As there is, in statistical terms, large-scale synchrony in the number of breeding goshawk pairs, brood size and timing of breeding over large distances, it is tempting to suggest that the synchronising agent for these parameters, the Moran effect (Moran 1953; a density-independent environmental

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2.4 2.8 3.2 3.6

2.4 2.8 3.2 3.6 (a)

B rood si ze in K ant a- H ä m e

Brood size in Pohjanmaa

16 18 20 22 24

16 18 20 22 24 (b)

Lay in g dat e in K ant a- H ä m e

Laying date in Pohjanmaa

Fig. 2.Correlations between annual (1989–1998) means of (a) brood size (r_s= 0.87,P= 0.001) and (b) laying date (r_s= 0.66,P= 0.04, 20 = 20 April) of goshawks breeding 140 km apart in Pohjanmaa and Kanta-Häme, cf.I. The larger circle in panel b indicaten= 2. A diagonal line indicating perfect spatial synchrony in brood-size and laying date is inserted in both panels.

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perturbation that correlates between locations), is working on at least a nationwide scale. As Finnish goshawks produce large broods in years when the local grouse density is high, whereas in lean grouse years, the broods are small (cf. Sulkava 1964, Tornberg and Sulkava 1991,II), one could conclude that this agent is the abundance of grouse. However, the correlations between grouse density and goshawk brood size varied considerably spatially, and the statistical strengths of most individual relationships were only moderate (>

80% of the correlation coefficientsr#0.4,II). Because the synchrony of grouse fluctuations decreases much more rapidly than the synchrony in goshawk brood size and breeding pairs, this implies that local grouse density is not the main factor influencing the number of breeding goshawk pairs and their realised breeding success over the whole of Finland. The same seems evident for timing of breeding, as goshawk laying date is not related to grouse density (Sulkava et al 1994, P. Byholm unpubl.). Concerning the number of breeding goshawk pairs, it is possible that the synchrony of this parameter furthermore is influenced by dispersal. As goshawk disperse longer distances than grouse (cf. Warren and Baines 2002,V), this could then explain why the synchrony in the number of breeding goshawk pairs levels off less steeply with distance than in the numbers of grouse.

Variation in nestling sex ratio

The observation that the spatial synchrony in grouse population dynamics falls off with distance is interesting as an almost matching pattern of decrease in synchrony was present in the data on local goshawk offspring sex ratios (III). Accordingly, nestling sex ratios in neighbouring areas were relatively well matching, whereas sex ratios of goshawk offspring in areas located far away had either nothing in common or they were even negatively correlated (Figs. 4a and 4b). This result resembles the patterns observed concerning seasonal sex-ratio shifts in the sense that the seasonal sex-ratio patterns varied a great deal if compared between distant sites. Consequently, sex ratio may de- cline, increase or remain unchanged as the season advances (I). This suggests

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Fig. 3.Spatial synchrony (cross correlation at lag 0) in (a) goshawk brood size during 1989–1998 when compared between 50 × 50 km grids (n= 28) and (b) in number of breeding goshawk pairs during 1989–2001 in 21 areas corresponding to national regional ornithological societies spaced all over Finland.

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that the sex ratio among goshawk offspring is a phenomenon of strictly local origin.

At a local scale (in 50 × 50 km grids) goshawk offspring sex ratio and grouse availability were highly correlated. In good grouse-years goshawks fledged more males, while in lean grouse-years offspring sex ratio was female biased (II). Because avian and mammalian predators are known to adjust their breeding effort in response to prey availability (e.g., Lack 1954, Newton 1998, Kokko and Ruxton 2000), this suggests that the sex ratio of goshawk offspring (one element of reproduction) is being locally shaped in response to food availability (cf. Edwards et al. 1988, Dzus et al. 1996, Appleby et al.

1997, Nager et al. 1999, Kalmbach et al. 2001). Since goshawk brood size to some extent correlated positively with local grouse density (see previous sec- tion), it was not very surprising that offspring sex ratio and brood size correlated positively in most grid units too. Large broods produced more males than females, whereas females were more common than males in small broods. This observation, i.e. that sex ratio differs between brood size catego- ries, is in line with patterns observed in several other size-dimorphic bird species (e.g. Howe 1977, Wegge 1980, Lindén 1981, Gowaty 1991, Anderson et al. 1993, but see Leroux and Bretagnolle 1996, Rosenfield et al. 1996). At first glance, it seems that resource availability affects nestling sex ratio in goshawks, large broods being male biased and small broods being dominated by females. However, a partial correlation analysis, reducing the effect of brood size on sex ratio, showed that the partialled-out correlation coefficients between goshawk sex ratio vs. grouse availability were consistently more often negative than expected by chance. This was not the case in partial correlations between sex ratio and goshawk brood size (II). In other words, irrespective of brood size, males are produced in good grouse years, females in years when grouse are scarce. The observation that the synchrony of both goshawk offspring sex ratio and grouse density changed in parallel (III) is then exactly what to expect. Furthermore, because the synchrony in grouse population fluctuations breaks down over larger scales (Ranta et al. 1995a,III), the rela-

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Fig. 4.(a) Correlation between offspring sex ratio (male proportion) in Pohjanmaa and Kanta- Häme (r_s= -0.14,P= 0.70), cf.Iand (b) spatial synchrony (cross correlation at lag 0) in goshawk offspring sex ratio when compared between 50 × 50 km grids (n= 28) during 1989–1998. Two dotted lines indicating an even sex ratio in Pohjanmaa and Kanta-Häme, respectively, are inserted with the figure (panel a).

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tionship between grouse and goshawk offspring sex ratio also diminished at scales larger than 50 km (III).

On the top of all this, if local offspring sex ratios in the end of the nestling stage were compared between subsequent years (sex ratio in yeartvs. sex ratio in yeart+ 1), negative correlations dominated the data (II). That is, if in one year the local offspring sex ratio is dominated by one sex (e.g., males), the next year there will be more of the other sex (females). This is what one could expect from frequency-dependent selection (Fisher 1930), or the homeostasis hypothesis (see Trivers 1985). Such a feedback mechanism, in concert with the fact that brood size did not correlate with sex ratio when adjusted for grouse density, could explain much of the observed inconsistency found in earlier local studies, where (within the same species) in some locations/years the proportion of males have been found to correlate positively with food abundance, but negatively in others (e.g. Edwards et al. 1988, Wiebe and Bortolotti 1992, Leroux and Bretagnolle 1996, Dzus et al. 1996, Korpimäki et al. 2000, Arroyo 2002).

In conclusion, it appears as if a Fisherian feedback in adjusting sex ratios is in action (cf. Lummaa et al. 1998, Ewen et al. 2001, but see Bensch et al.

1999, Szczys et al. 2001), in concert with spatial coupling of population sub- units (Ranta et al. 2000). At the same time, however, sex ratios of offspring goshawks are related to local grouse availability, which influence the mother’s condition. In this scenario female condition would thus be the mechanism that proximately shapes goshawk offspring sex ratios (e.g. Nager et al. 1999, Kalmbach et al. 2001, Sutherland 2002).

Variation in clutch size, partial-brood loss and egg sex ratio

Average goshawk clutch size differed significantly between years (1999–

2002), as did also the relative abundances of different sized clutches (IV).

Like in many other raptors (Newton 1979), this variation was partly parallel to local prey density. Larger clutches were more common in good grouse regimes than in lean ones (Sulkava et al. 1994,IV). Yet clutch size variation showed little general variation. Three- and four-egg clutches made up 90% of the data, while two-egg clutches (7%) or five- and one-egg clutches (3%) were rare. This frequency distribution does not correspond to that of broods (Chi-square test,c²₄= 65.3,P< 0.00001; data from Pohjanmaa during 1999–

2002, Fig. 5), and in the main study area there is more variation in average an-

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1 2 3 4 5

0 20 40 60 80 100 120 140 (a)

Numbersofclutches

Clutch size

1 2 3 4 5

0 20 40 60 80 100 120 140 (b)

Numbersofbroods

Brood size

Fig. 5.The frequency distribution of (a) goshawk clutch size and (b) brood size during 1999–2002 in the main study area.

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nual brood size (38% difference between worst and best year, cf.I) than in average annual clutch size (9%, cf.IV). In other words, the number of goshawk nestlings (I,II) only occasionally matches the initial clutch size; the brood is often smaller than the clutch.

The factor causing this discrepancy is partial brood-loss: the number of offspring lost through partial brood-losses clearly varied on an annual basis, although not linearly with clutch size. Partial brood-losses hit roughly two times harder in four-egg clutches than in clutches of two, three or five eggs (IV). Partial brood-loss was also related to grouse abundance, and interest- ingly, inversely so; as compared between two regions with different grouse densities partial brood-loss was more common where grouse were abundant than where they were scarce (IV). At the same time, variation in partial brood- loss patterns was also observable at the level of individual territories (n= 123). It is thus clear that partial brood-loss in goshawks is a phenomenon of multiple causes (cf. Forbes et al. 2002), and observable at several spatial scales (territorial–regional).

Together, these patterns of clutch size and partial brood-loss variation in- dicate that goshawks typically lay an optimistically large clutch size (four eggs), especially in good grouse regimes, but in general the success of this op- timistic family size is low. However, occasionally clutches of four also suc- ceeded (IV), and in this sense, the pattern observed support the traditional

“brood-reduction hypothesis” (Lack 1954, Temme and Charnov 1987, Pijanowski 1992), which postulates that parents produce an optimistically large clutch size in order to be able to capitalise on unpredictable favourable conditions during the nestling period. If the production of an ’extra’ fourth egg is energetically cheap (Horsfall 1984, Carey 1996), especially in times/areas with many grouse, goshawks might have little to lose in producing a risky fourth egg even if they could not anticipate the density of addi- tional (migratory) prey species in the nestling period, possible of crucial im- portance for the nourishing of young. Indeed, goshawks switch from a diet clearly dominated by grouse in the early breeding season to a more diverse one during the nestling stage, when migratory bird species, such as thrushes, form a large part of the diet (Lindén and Wikman 1983, Widén 1987, Tornberg 1997).

Albeit the local average population-level sex ratio among goshawk nestlings differs considerably between years (I, II), male offspring (the smaller sex) dominated the data if it was analysed over several years/regions (I,IV). A similar pattern has been observed in several other size-dimorphic raptors as well (e.g., Edwards et al. 1988, Zijlstra et al. 1992, Leroux and Bretagnolle 1996, Rosenfield et al.1996, but see Picozzi 1980, Arroyo 2002). Interest- ingly, even though partial brood-loss largely reduced clutch size, secondary family-size adjustment (IV) did not significantly add to the sex ratio among goshawk nestlings (cf.I,II,III). This becomes evident as, (a) potential sex- biased egg mortality did not alter the sex ratio significantly, and (b) offspring sex ratios did not differ between nests facing and not facing partial brood-loss (IV). This again indicates that the sex ratio observed among goshawk nestlings mainly is the results of facultative manipulation of egg sex (cf. Appleby et al. 1997, Komdeur et al. 1997, Nager et al. 1999, Sheldon et al. 1999, Kalmbach et al. 2001). Since young goshawk males suffer higher mortality during their first two years of life than young female goshawks (Kenward et

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al. 1999), production of more males under good main prey (grouse) regimes could be adaptive. This conclusion seems especially firm when taking into account the fact that goshawk males may have few alternative prey but grouse during the harsh Finnish winter, than what the larger females have, which are capable of killing larger prey species (e.g. hares) if grouse are not available (Tornberg 2000, Tornberg and Colpaert 2001).

Dispersal

The analysis of goshawk ring recoveries (V) showed that the decision to leave or to stay in the natal area (a 50 × 50 km grid) made by juvenile goshawks in their first winter was strongly related to hatching date. Individuals hatched in the earliest quarter of the hatching-date range were less prone to leave the natal area than individuals hatched in the last quarter of the range. Only c. 30%

of early-hatched birds left the natal area, whereas almost 100% of birds hatched only 30 days later dispersed to another 50 × 50 km grid than the one initially hatched in. In addition, young hatched first within a brood (first- ranked) also tend to stay local more often than their later-hatched siblings in the same brood (V). Sex did not add to either of the above patterns.

However, in general juvenile females dispersed shorter distances during their first winter than did juvenile males. Hatching date added to this behaviour, but only to that of male offspring. Early-hatched males dispersed shorter distances than late-hatched ones, whereas the dispersal distance of juvenile females was unrelated to hatching date. On the top of all this, grouse density in the natal area influenced how long a juvenile goshawk dispersed during its first winter. In times and places when/where local grouse density was high, goshawks dispersed shorter distances than when local grouse availability was low (V, Fig. 6). This held for both males and females, and fits well with patterns observed in raptors in general, including goshawks (Kenward et al.

1993b, Adriaensen et al. 1998, Kennedy and Ward 2003).

Summary 17

-1 0 1 2

3.2 3.6 4.0 4.4 4.8

D is per sa ld is tanc e (log)

Residual grouse density

Fig. 6.Dispersal distance of juvenile goshawks during their first winter (Octo- ber 1–February 28) as es- timated from ring recoveries in relation to de- trended residual grouse density (seeVfor details) in the natal area (a grid sized 50 × 50 km) during 1989–2000.

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Pooled, the results suggest that juvenile goshawks distribute themselves in space according to an ideal despotic distribution process (Sutherland 1996), where early-hatched and first-ranked birds are in a superior position in competition for local winter territories. This competition seems to be more pronounced among juvenile males than among juvenile females (V), but the competition is relaxed in good grouse years. In general, the results support theoretical models arguing that competition for resources is the primary force driving dispersal in birds and mammals (e.g., Waser 1985). Clearly, this competition is present on several levels of organisation (individual, brood, landscape), and in addition the response may also differ between the sexes.

On the other hand, in adult birds (+3 years), none of the analysed variables were related to dispersal distance, whereas the probability of remaining local tended to be related to local grouse density in the hatching year (for males only). If the dispersal distances between juveniles and adults were compared, simultaneously accounting for the effect of sex, neither the effect of sex nor age were solely related to the distance travelled, whereas their interaction was highly significant (V). Since juvenile males disperse farther from the natal area than juvenile females, this effect arises as males are recovered closer to their natal site as adults than they are as juveniles. Assuming that young goshawks prospect wide areas before settling down to breed, as do several other raptors (Newton et al. 1994, Walls and Kenward 1998, Forero et al. 2002), this suggests that whether an adult goshawk male will return to its natal area to breed or not is related to whether the area inhabits a high enough number of grouse. Nevertheless, factors that add to the dispersal behaviour of juveniles are not the same that add to the dispersal of adults. It is therefore important to study dispersal behaviour of juveniles and adults separately in long-lived species (Kenward et al. 2001) experiencing differing environmental conditions (in multiple areas) as nestlings.

Conclusions

I think there are at least four lessons to learn from the spatially and temporally variable patterns of reproduction and dispersal found in goshawks described in this thesis. First, the variation in all studied parameters (clutch size, brood size, offspring sex ratio, partial brood-loss and dispersal) is influenced by multiple factors. Even if this perhaps can seem trivial, it is important to iden- tify what these factors are for any species, as this information might be vital, e.g., for optimising conservation or wildlife management acts. Indeed, there is concern about the status of goshawk populations in parts of the goshawk’s range (e.g. Crocker-Bedford 1990), whereas it is considered a problematic predator on game birds in other areas (e.g. Kenward et al. 1981, Selås 1997).

Especially dispersal is poorly known for most vertebrates (Clobert et al.

2001), although clearly essential for population persistence.

Second, it is important to acknowledge that the influence of these factors is not unambiguous when compared between different sites (territories–larger areas) even in the same time window (between years, within seasons). This is especially profound for goshawk sex ratios and dispersal. Thus, conclusions made from patterns of sex ratio variation observed in a single-site study might

18 Byholm, P.

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easily be premature, especially if the probability of recruitment differs regionally between females and males (Arroyo 2002). Even if not examined yet, the fact that both goshawk offspring sex ratio and dispersal are linked to grouse density, in combination with the fact that goshawk offspring sex ratios are spatially coupled, suggests that recruitment probability of goshawk males and females could differ regionally.

Third, analysing patterns over wide areas tells us something about the scale at which elements of interest are shaped. For example, the number of breeding goshawk pairs, average goshawk brood size and laying date seem less locally influenced (varies in synchrony over the whole of Finland) than goshawk offspring sex ratio and local grouse density (varies in clear synchrony over a few tens to hundred kilometres only). This does not mean that parameters which are in synchrony over large areas, e.g. goshawk brood size, would not also be determined by local factors (e.g. territory-dependent partial brood-loss), but certainly it suggests that these small-scale factors are affecting large areas similarly. Finally, because reproductive success (especially offspring sex ratio) and dispersal differ between locations, ignorance of the spatial dimension can constrain our understanding of the dynamics of populations (e.g. Ranta et al. 1997b, Tilman and Karieva 1997, Kendall et al. 1999, Ranta et al. 1999, Kokko and Ruxton 2000) as population-entities are not isolated from each other. Equally, ignorance of space could confound measures of population growth, expansion and viability in species where locations of varying quality are occupied over long time periods (Sutherland 1996, Kokko and Sutherland 1998, Krüger and Lindström 2001, Thompson et al. 2001, Ambrosini et al. 2002).

These results point at a need to incorporate the spatial dimension more rigorously into future life-history studies. Even though the repercussions on goshawk population dynamics produced by the variable patterns of dispersal and reproductive success still have to be clarified, the material presented in this thesis certainly could work as an incentive for studies addressing such questions.

Acknowledgements

First of all I would like to thank my supervisor Hannu Pietiäinen who has always had the time for all kind of discussions during the years, either related to my work with this thesis directly or concerning more general topics. In fact, without Hannu’s multi-annual data on the size of Ural owl eggs, which I analysed in my MSc-thesis, my fascina- tion for how a variable environment relates to reproductive parameters in birds could possibly never have come to flourish. I am very glad that you gave me the opportunity to study your data back in the 1990’s – this thesis is much an upshot of that. I also owe you great gratitude for constructive criticism towards many of my initial manuscripts that sometimes, perhaps, have been a bit too ’bold’. Second, I am very grateful to Harto Lindén who, I feel, in many respects almost has functioned as a second supervisor. You have been encouraging me in my work in many different ways under the years, and both you and Marcus Wikman have always responded positively to my re- quests for material on grouse from the Finnish Game and Fisheries Research Institute.

Many thanks! I would also like to thank Esa Ranta who once back in 1999 had the time to listen to my ideas of how one could potentially make use of the large-scale material on goshawks and grouse. Indeed our co-operation has been both interesting and fruit- ful, and it has certainly further convinced me that the spatial dimension is something

Summary 19

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important in ecology not to be forgotten. Without Pertti Saurola’s positive attitude towards me using the huge amount of data available at the Ringing Centre this study would not have been possible. Thank you for that! I also wish to thank the rest of my co-authors Jon Brommer and Veijo Kaitala – working together with you has been a pleasure. Science is definitely something best done together. In addition, my warmest thanks go to Ilpo Hanski, Jyrki Holopainen, Juha Meilä, Bob O’Hara, Hannu Rita, Paul Stevens, Jari Valkama and Stephen Venn for commenting on and otherwise helping me with my manuscripts, hereby greatly improving their quality.

I have had the joy to spend time many hours in the field with members of the Suupohja raptor-team. Many thanks to Ivar Hagback, Jens Hagback, Jari Her- nesniemi, Kari Ketola, Harry Lillandt, Canne Lundberg, Ismo Nousiainen, Kari Palo, Jukka-Pekka Taivalmäki and Ville Yli-Teevahainen – without the information origi- nating from ’your’ goshawk nests the material collected in the field would have been much more unpretentious than what it is. A special thanks to Canne for originally in- troducing me to the world of bird ringing in the 1980’s and making me realise that “en riktig karl ska lukta skit och svett” while conducting field-work! Per-Erik Byholm, Bo-Göran Lillandt, Niclas Fritzén, Mikael Norrteir, Kenneth Rosenlund and Lars Söderback are acknowledged for helping with a variety of field-work related matters.

The whole personnel at the Ringing Centre have always had the time for the most ob- scure and laborious work of all kinds whenever prompted – my warmest thanks to Heidi Björklund, Jukka Haapala, Päivi Kare, Seppo Niiranen, Pekka Puhjo and Jarmo Ruoho. I like to thank Jodie Painter and Craig Primmer for co-operation and help while occasionally being in the lab. Pekka Helle and Ari Nikula have functioned as collaborators up north – thank you for your effort this far; we have good science going on. The pre-examiners of the thesis, Hanna Kokko and Mikko Mönkkönen did a great job in improving the final draft of the work.

Back at the department I wish to thank my ’room-mates’ Mikko Kolkkala, Jukka T. Lehtonen and Tuomo Pihlaja for a peaceful work-environment. Without the com- pany of Jon Brommer, Marianne Fred, Nuutti Kangas, Patrik Karell, Matti Koivula, Anne Luoma, Tomas Roslin, Teija Seppä, Markus Öst and many others the comple- tion of this work would have been much less enjoyable than what it has been. What would life be without a good cup of coffee every now and then together with a bunch of nice fellow-workers? My mother Kristina and my father Per-Erik are acknowledged for never hindering me to climb in any tree when I was a kid; without this ’li- cense to climb’ I am sure that the goshawks breeding around the village of Pjelax would never have been a part of science whatsoever. Last but far from least, I would like to thank my loved wife Leena for all warming words and support throughout the years. Just knowing that you are there whenever I might need you has helped me in my work at several occasions. Our children, Benjamin and Matilda, are thanked for reminding me of that making science is not the only assignment here in the world. After a day of hard work, few things are as reminding of this as two berserkers that rush into ones arms as soon as one dare to step inside the door…

This work would of course not have been possible without funding. I am grateful for the financial support from the Ministry of Education (LUOVA graduate school), Otto A. Malms donationsfond, Waldemar von Frenckells stiftelse, Oskar Öflunds Stiftelse, Suomen Riistanhoito-Säätiö, Svenska Kulturfonden, Svensk-Öster- bottniska Samfundet and Jenny ja Antti Wihurin rahasto.

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Summary 23

Reproduction and dispersal of goshawks in a variable environment

Reproduction and dispersal of goshawks in a variable environment

Patrik Byholm

Reproduction and dispersal of goshawks in a variable environment

Patrik Byholm

Contributions

Contents

Summary

Introduction

The goshawk

Material

Results and discussion

B rood si ze in K ant a- H ä m e

Lay in g dat e in K ant a- H ä m e

D is per sa ld is tanc e (log)

Residual grouse density

Conclusions

References