Tawny Owls Strix aluco with reliable food supply produce male-biased broods

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1 Ibis (2007), 149, Blackwell Publishing Ltd Tawny Owls Strix aluco with reliable food supply produce male-biased broods KASI B. DESFOR, JACOBUS J. BOOMSMA & PETER SUNDE* Department of Population Biology, Institute of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark Tawny Owls Strix aluco have been reported to skew the sex ratio of their offspring towards males when facing food shortage during the nestling period (and vice versa), because female fitness is more compromised by food shortage during development than male fitness. To test the generality of these results we used a DNA marker technique to determine the sex ratio in broods of Tawny Owls in Danish deciduous woodland during two years of ample food supply (rodent population outbreak) and two years of poor food supply. Of 268 nestlings, 59% were males (95% CI: 53 65%). This proportion was higher than previously reported for the species (49% in Northumberland, UK, and 52% in Hungary), but consistent with Fisherian sex allocation, which predicts a male bias of c. 57% based on inferred differences in energy requirements of male and female chicks. Contrary to previous results, brood sex ratios were not correlated with the resource abundance during the breeding seasons, despite considerable variation in breeding frequency, brood size or hatching date across years. Brood sex ratios were unaffected by brood reduction prior to DNA sampling, and nestling mortality rates after DNA sampling were not related to gender. The inconsistency between the sex ratio allocation patterns in our study and previous investigations suggests that adaptive sex allocation strategies differ across populations. These differences may relate to reproductive constraints in our population, where reproductive decisions seem primarily to concern whether to lay eggs at all, rather than adjust the sex ratio to differences in starvation risk of nestlings. Offspring sex ratios are predicted to be optimized so that parental fitness is maximized despite variation in environmental and social conditions (Pen & Weissing 2002), but not all empirical data in birds confirm these expectations (e.g. Wiebe & Bortolotti 1992, Bensch et al. 1999, Cockburn et al. 2002). This could be due to sex allocation models for birds not taking all relevant complexities of life history and physiology into account (Komdeur & Pen 2002, Komdeur et al. 2002). Because sex allocation patterns may be different across populations of the same species, comparative studies from different locations are important for understanding the selective pressures on sex allocation and to unravel the adaptive basis for observed sex ratio biases (Krackow 1993, 2002). To clarify whether a conclusion about adaptive relationships drawn from an initial study *Corresponding author. Present address: NERI Department of Wildlife Ecology and Biodiversity, Grenåvej 14, DK-8410 Rønde, Denmark. psu@dmu.dk has general validity for the species or merely reflects a local adaptation or even a spurious correlation (publication bias), much could be learned by conducting multiple studies from different populations facing different environmental conditions. Birds of prey with fluctuating food supply, reversed sexual size dimorphism (females being larger than males) and asynchronous hatching provide interesting models to test sex allocation strategies under varying environmental conditions. Variation in egg or nestling sex ratios appears to be widespread among owls, raptors and skuas (e.g. Appleby et al. 1997, Kalmbach et al. 2001, Arroyo 2002, Byholm et al. 2002, Hipkiss et al. 2002, Brommer et al. 2003). Tawny Owl Strix aluco females are 20% heavier than males (Sunde et al. 2003). Assuming that smaller body size makes male offspring cheaper to produce for the parents, we thus expect males to be numerically over-represented at the population level to a degree that is inversely proportional to the ratio of typical production costs of a male and female offspring (Fisher 1958, Pen &

2 Male-biased nestling sex ratios in Danish Tawny Owls 99 Weissing 2002). In addition, Trivers and Willard (1973) predicted that variation in parental quality (possibly in response to environmental or social conditions) would make it adaptive for parents to manipulate their sex ratio at the clutch level. Two previous studies have found support for the Trivers & Willard hypothesis in the Tawny Owl (Appleby et al. 1997, Sasvári & Nishiumi 2005). In Kielder forest, Northumberland, Appleby et al. (1997) found an overproduction of females under favourable food conditions, whereas more males were produced in territories facing poor food conditions in a given year. At the population level, the overall sex ratio was close to parity (49% males). Because prey availability either improved or deteriorated during the breeding cycle, Appleby et al. (1997) argued that egg-laying mothers could predict food supply several months before the demands of their clutches became maximal. Because female nestlings appeared to suffer more than male nestlings when raised under poor food conditions (affecting fertility in adults), it appeared to be an adaptive strategy to skew the brood sex ratio towards females if prey densities were increasing and towards males when prey declined. In a Hungarian forest, Sasvári and Nishiumi (2005) found significantly more males (79%) among the first hatched young in years of difficult feeding conditions (permanent snow cover) than in years with benign foraging conditions (42%), whereas no difference existed for second and third hatched young. In total, 52% of the investigated hatchlings were males. The conditional variation in hatchling sex ratio appeared to be adaptive as younger siblings of the female sex suffered disproportionately high nestling mortality in snowy years. Contrary to the results of these two studies, Brommer et al. (2003) found no evidence of brood sex ratios being adjusted to the breeding situation during three years of low prey supply in the closely related Ural Owl Strix uralensis in Finland. Instead they found that the overall sex ratio was significantly male biased (56%). As nestlings were only sampled in years of poor food supply it remained unclear whether this male-biased sex ratio reflected the population mean or a conditional response to poor breeding situations. We obtained sex ratio data for 268 Danish Tawny Owl nestlings from 68 broods, sampled over four years with considerable variation in food supply (alternating between high and low rodent densities). We first tested whether the offspring sex ratio followed the constraints imposed by random Mendelian segregation producing a 50 : 50 primary sex ratio, or was biased in the direction of the presumably cheaper male sex. Next, we analysed whether a series of conditional variables had any effect on the clutch-specific sex ratios as has been observed elsewhere (Appleby et al. 1997, Sasvári & Nishiumi 2005). To evaluate the general validity of Sasvári and Nishiumi s (2005) observation that conditional responses in sex ratio were only expressed in the first hatched chick (higher proportion of males among the first hatched young in years of severe feeding conditions), we also checked whether the male sex was particularly over-represented among the first hatched young in poor years. Variation in sex ratio may also occur because of differential mortality at the nestling stage (e.g. Nager et al. 1999, Arroyo 2002, Hipkiss et al. 2002), so that parental manipulation of the primary sex ratio is invalid as an exclusive explanation. To control for possible effects of sex-specific mortality (see Fiala 1980), we tested whether brood sex ratio was affected by differential nestling mortality between oviposition and DNA sampling and between DNA sampling and fledging, both in the good and in the poor years. METHODS From 2000 to 2003, DNA from Tawny Owl nestlings was sampled in Northern Zeeland, Denmark (55 N, 12 E; see Sunde et al for a further description of the habitat). Yearly fluctuations in prey abundance were due to mast years of Beech Fagus silvatica, which produced alternating low (breeding seasons of 2000 and 2002) and high (2001 and 2003) densities of small rodent prey the following spring. Trap indices from 1998 to 2000 indicated that the catches of small mammals (which comprise > 90% of the Owls diet in good breeding seasons) were at least three times higher in good years than in poor years. In both types of years, the trap indices increased by a factor of 6 10 from May to August (Bølstad 2001, P. Sunde unpubl. data), suggesting a seasonal increase in food supply in good as well as poor seasons. The study area contained Tawny Owl territories or 230 nesting sites (nestboxes plus natural cavities) that were systematically searched for breeding each spring. In 20 of the territories, reproductive success was systematically monitored in by controlling all accessible nesting places for breeding attempts. Additionally, undetected breeding attempts were searched for by nightly surveys, listening for

3 100 K. B. Desfor, J. J. Boomsma & P. Sunde food cries of fledged young (Sunde & Markussen 2005). Incubating females were usually caught by putting a net in front of the nest entrance and were put back in the nest after being weighed. Nestlings were age-estimated with a maximum inaccuracy for both sexes of ±2 days (95% confidence limits around the individual observation) using sex-specific growth curves for the 4th primary (P. Sunde unpubl. data). Between 8 and 34 days after hatching (median = 19 days), µl of blood was sampled for DNA analysis from the brachial vein, and stored in dimethyl sulfoxide (DMSO). DNA was extracted from the blood samples (50 µl) using the DNeasy protocol and tissue extraction kit. Two hundred microlitres of water (double distilled H 2 O) were used to dilute the extraction, after which half the sample was stored at 20 C and the other half at 4 C for immediate use. Sex was determined with the protocol of Fridolfsson and Ellegren (1999), based on PCR amplification of two sex-linked introns (CHD1W and CHD1Z) with a specific set of primers (2550F and 2718R). All reactions were performed in 10-µL volumes containing 0.05 U of AmpliTaq (Perkin- Elmer), 200 µm of dntps, 10 mm Tris-HCl, ph 8.3, 17.5 mm MgCl 2 and 2 pmol of primers 2550F (5 - GTTACTGATTCGTCTACGAGA-3 ) and 2718R (5 -ATTGAAATGATCCAGTGCTTG-3 ). All reactions were run on a Hybaid express thermal cycler using a touchdown procedure (Don et al. 1991) with the following settings: an initial denaturing step for 2 min at 94 C, followed by an 11-cycle touchdown with 30 s denaturing at 94 C, an annealing range of C (1 C reduction per cycle) for 30 s, and 30 s extension at 72 C. An additional 25 cycles of 94 C for 30 s (denaturing), 50 C for 30 s (annealing) and 72 C for 30 s (extension) were run to complete the amplification after a final 5-min extension at 72 C. Products were separated on 3% agarose gels and bands were detected with ethidium bromide staining and measured against an X174 ladder (NEB). Because the two introns represent the avian W and Z chromosomes, a heterogametic female always showed two bands (WZ), whereas a single band (ZZ) characterized a male. Sex ratio variation was analysed with logistic regression (PROC GLIMMIX in SAS 9.1, SAS Institute). Observations were entered per clutch and deviances were scaled for overdispersion (McCullagh & Nelder 1989, Boomsma & Nachman 2002) or individually while adjusting for between-brood variation by entering brood as a random effect (Krackow & Tkadlec 2001). The observed overall sex ratio was tested both against the expectation of parity (50 : 50) and against the ratio predicted by an even energy allocation to the dimorphic sexes. Lacking data on sex-specific energy requirements of raising a male or female Tawny Owl offspring to independence, we used the relative difference in adult lean body mass as a proxy for the assumed difference in energetic requirements over the entire breeding season (young Tawny Owls have reached full adult size when parental care stops /2 months after fledging). Adult body masses were obtained from 18 breeding males (mean ± sd, 402 ± 23.0 g) and 21 females (524 ± 44.0 g) captured when their young were days old, i.e. when the body masses of parents reached their seasonal low (no stores of metabolically inactive fat: P. Sunde unpubl. data). Assuming that the energetic expenditure of producing offspring is approximately proportional to mass (Vedder et al. 2005), we inferred that males require 77% of the total energy of females. This implied that the expected numerical sex ratio under equal investment in the sexes was approximately 56.6% male assuming that male and female offspring had equal chances of surviving until independence. The effects of variables specifying the quality of the breeding situation (year, type of year, date of hatching, clutch size, female mass during incubation of eggs) on brood sex ratio were assessed based on the reduction in scaled deviance ( D) after adding each individual factor to a baseline model that only included the constant (intercept). The same procedure was applied when testing whether brood sex ratio was affected by gender-specific nestling mortality prior to DNA sampling (measured as the presence or absence of brood reduction and the proportional reduction of the brood between hatching and sampling). In the presence of differential mortality prior to sampling, reduced broods should contain a larger proportion of the sex with greater survivorship (Fiala 1980). Finally, we tested whether the sex of the offspring depended on the relative age (in days after hatching of the oldest nestling) or on hatching order within the brood. As Sasvári and Nishiumi (2005) found that conditional sex ratio variation was only present among the first hatched chick in a brood (higher proportion of males among the first hatched young in years of severe feeding conditions), we analysed whether the male sex was particularly over-represented among the oldest young in poor years. Unlike Sasvári and Nishiumi s (2005) study, we could not always identify the first hatched chick in the broods owing

4 Male-biased nestling sex ratios in Danish Tawny Owls 101 Table 1. Reproductive data from broods where the young were sexed. Date of hatching Female mass at incubation Clutch size Year Mean sd n mean sd n mean sd n April March April March Tests of factors F df P F df P χ 2 df P Year ,82 < , < Type of year ,84 < , < Type of year refers to level of food availability (poor = 2000, 2002; good = 2001, 2003). Differences in clutch size were tested by Kruskal Wallis tests as homogeneity of variances could not be achieved. to the uncertainty of 2 days within the age estimation. Instead of comparing sex ratios between the first hatched young and the younger chicks in relation to the food condition, we treated the estimated age hierarchies as a covariate in our analysis. Gender-specific mortality from DNA sampling to fledging was inferred from Mayfield logistic regression (Aebischer 1999, Hazler 2004). After DNA sampling, nests included in the nestling survival analysis were visited once every 3 6 days. Most of the nestlings in the analysis (110 of 153) were radiotagged prior to fledging so that exact fledging dates were available. Nests of untagged young were revisited after the breeding season in order to retrieve any rings of deceased nestlings. Surviving young from these nests were assumed to have fledged at the age of 32 days. Causes of nestling mortality were categorized as being due to disease (carcass not malnourished, body mass > 250 g), outright starvation (extensive malnourishment, body mass < 250 g) and unknown causes (typically only the ring was found). Daily mortality was modelled as the number of deaths per exposure day as a function of various fixed (gender, days since hatching, quality of year, etc.) and random effects (brood ID) by means of a generalized linear mixed model (PROC GLIMMIX with a binomial error and logit link function in SAS statistical software). Each survey interval per chick was entered as an observation (e.g. if a chick was DNA sampled when 24 days old and observed alive at a next nest visit 4 days later, but found dead at the following nest visit 3 days later, the data were entered as two observations with response variables 0/4 and 1/3 events/trials, respectively). The effect of sex on mortality risk was tested with a model including the effects of age (at the start of the survey interval), relative age (in relation to oldest chick in the brood) and the quality of the breeding season. Because most deaths were due to disease, mortality rates were also analysed in relation to causes other than disease. All statistical tests reported are two-tailed. RESULTS Annual variation in quality of breeding situation The breeding frequency was highly variable across years: of 20 pairs that were carefully assessed for reproduction through all 4 years, 25, 100, 50 and 90% laid eggs during 2000, 2001, 2002 and 2003, 2 respectively ( χ 3 = 12.61, P = 0.006; difference between good years of rodent population outbreaks 2 (2001, 2003) and poor years (2000, 2002): χ 1 = 11.08, P = ). In good years, incubating females were heavier and produced clutches that were about twice as large and that hatched 1 month earlier than those in poor years (Table 1). Sex ratios and survival of nestlings Of 268 nestlings sampled from 68 broods during the four years, 159 were males (Fig. 1). Even though the residual deviance tended to be overdispersed (Deviance/df = /85 = 1.39), a randomization test suggested that the sexes were distributed evenly across the broods (P = 0.30). There was also no significant effect of brood ID if entered as a random factor (z = 0.46, P = 0.76). The 95% confidence intervals around the observed numerical proportion of males (0.593) were and when adjusted for overdispersion. Both these sex ratio

5 102 K. B. Desfor, J. J. Boomsma & P. Sunde Table 2. Tests of factors that possibly influence brood sex ratios, given as reduction in scaled deviance ( D) after adding the factor to a model that only included the intercept constant (as no factors were significant, the resulting model only included the baseline function). Classes of variables Factors k n m D df P B n* Brood attributes Year ( ) Quality of year (good vs. poor) Date of hatching (Julian days) Date of hatching (residual to year) Clutch size Female mass during incubation Brood reduction (indicator) Brood reduction (proportional) Individual attributes Relative age within brood (A) Hatch rank A Quality of year Relative age within a brood was entered as log(age difference in days to oldest young + 1). k = number of broods, n = number of nestlings, m = number of males. The potential influence of the different factors on the sex ratio is indicated with their parameter values (B; a positive value indicates that the factor had a positive effect on the proportion of males) and as the approximate number of young 2 required in a statistical analysis for the observed effect to be significant (n* = n( χ α=005 / D) given it was factual).. Sex ratios were independent of the type of year, laying date, clutch size or the mother s condition during incubation (Table 2). Within the broods, the proportion of females was slightly, but not significantly, higher among first hatched young than later hatched siblings. In contrast to the Hungarian study (Sasvári & Nishiumi 2005), there was no interaction between the quality of the year and the sex ratio in relation to hatching order. Other possible predictors of the brood sex ratio were not significant, but would often have required very large sample sizes to reach significance (Table 2). This suggests that the biological effects of these factors were indeed minor, rather than the statistical power of the tests being too low. Figure 1. Sex ratios (± 95% confidence limits) of Tawny Owl nestlings during the four years of sampling ( ). estimates were significantly different from parity ( D 1 = 9.22 and 6.65, P = and , respectively), but not from the estimated proportion expected under equal resource allocation to the sexes (56.6% males: D 1 = 0.87 and 0.59, P = 0.37 and 0.44, respectively). The overall proportion of males was significantly higher than previously reported for the Tawny Owls in Kielder Forest (81 males, 83 females: Appleby et al. 1997, randomization test: P = 0.049) and tended to be higher than the proportion of males observed in Hungary (93 male and 87 female hatchlings: Sasvári & Nishiumi 2005, P = 0.12). Tests for sex-specific nestling mortality There was no evidence of sex-specific mortality prior to sampling, as the sex ratios of broods where no nestling mortality had occurred prior to DNA sampling were similar to those of broods where one or more nestlings had died (Table 2). In 47 nests initially controlled at the incubation stage, 40 out of 178 eggs (23%, 95% CI: 16 31%) died before DNA was sampled, independent of the type of year ( D 1 = 1.10, P = 0.29). At least 17 of the 40 eggs that died never hatched. Survival data from DNA sampling to fledging were obtained from 153 chicks (52 broods) surveyed over a total of 1550 exposure days. Within this interval, 16 young (1.0% per day) died of disease not related to food limitation, six (0.4% per day) of unknown causes and three of outright starvation (0.2% per day). The

6 Male-biased nestling sex ratios in Danish Tawny Owls 103 Table 3. Gender-specific mortality rates of nestlings from DNA sampling to fledging analysed with Mayfield logistic regression in a generalized linear mixed model. All deaths: 25 events (brood: Z = 1.89, P = 0.06) Deaths not due to disease: 9 events (brood: Z = 1.58, P = 0.11) Fixed effects df F P B df F P B Included in model: Constant Quality of year (poor) 1, , Days since hatching 1, Position in age hierarchy 1, Additive effects of sex: Sex (male) 1, , Sex + sex quality of year 2, , To adjust for clustering of fates within broods, brood ID was entered as a random effect. The hypothesis that survival is gender-specific in general or in interaction with other factors was tested as additive effects to a model including the effects of age (days since hatching), the quality of the breeding season (poor (2000, 2002) vs. good (2001, 2003)) and position in the age hierarchy (log(1 + age difference in days to oldest sibling)), which appeared to be the factors that best explained mortality of nestlings. The mortality risk per day predicted from a model s parameters (B) is given as: M = exp(λ)/exp(λ + 1), where λ is the linear predictor (λ = B 0 + B 1 x 1 + B n x n ). total mortality rate was higher in poor years than in good years (2.4 vs. 1.2% per day), but was not in any way related to gender (Table 3). The mortality rate attributed to reasons other than disease also tended to be higher in poor years than in good years (1.1 vs. 0.4% per day), but was independent of gender (Table 3). DISCUSSION Brood sex ratios were not affected by mortality prior to DNA sampling. The observed nestling sex ratio of 59% males should therefore be similar to the sex ratio at laying (Fiala 1980), unless female offspring died at a disproportionately much higher rate prior to DNA sampling in female-biased broods. However, we consider female-biased mortality in female-biased broods to be unlikely as the sole explanation for the observed male bias at the population level, as only 23% of the eggs died before being DNA sampled (of which nearly half were known not to have hatched at all). Moreover, if female offspring suffered increased mortality in female-biased broods, this would result in lower sex ratio variation across broods than expected by chance (i.e. in underdispersed binomial errors), which was not the case (binomial errors were slightly overdispersed). We therefore infer that sex ratios at sampling probably reflected sex ratios at laying quite well. Likewise, there is no reason to believe that the offspring sex ratio was any less male biased at cessation of parental effort /2 months after fledging than during sampling in the nest. Hence, mortality was equal for both sexes from sampling to fledging. From fledging to independence, males survived somewhat (although not significantly) better than females (Sunde 2005). In the absence of sex-specific mortality throughout the period of parental investment, higher energetic rearing costs of female offspring appear to be the most likely adaptive explanation for the apparent overproduction of male offspring in the present population. The proportion of males was close to the proportion (57%) predicted from Fisherian sex allocation theory, assuming that the estimated difference in energetic requirements is proportional to the difference in lean body mass of full-grown offspring. The overall proportion of males appeared to be higher than in the Tawny Owl populations in Northumberland (49%: Appleby et al. 1997) and Hungary (52%: Sasvári & Nishiumi 2005), which may imply that the overall cost ratio of producing male and female offspring varies across populations. No variation in the brood sex ratios was explained by factors indicating variation in the breeding situation, as would have been expected when parents adjust their sex allocation strategy to environmental or social conditions (Trivers & Willard 1973). In particular, we could not confirm that more females were produced in favourable food situations (Appleby et al. 1997, Sasvári & Nishiumi 2005), i.e. in mast years, in early hatched broods or when mothers were well fed as should be expected if this sex was overproduced under favourable resource conditions.

7 104 K. B. Desfor, J. J. Boomsma & P. Sunde Our results differ from the previous two studies of sex allocation in Tawny Owls by Appleby et al. (1997) and Sasvári and Nishiumi (2005), both at the population level (higher total proportion of male offspring) and at the brood level (lack of conditional sex allocation), and apparently also at the within-brood level (no interaction between breeding conditions and sex ratio in relation to hatching order). The last was found by Sasvári and Nishiumi (2005) although the statistical treatment in their study was slightly different from ours. As our sample is the largest data set on the species to date and the observed effects would have required even larger sample sizes to achieve significance had they been true, we feel confident that these non-significant results (which had been formulated a priori based on the results of these previous investigations) were not due to mere lack of statistical power. The mismatch between our results and the previous results should therefore have a reason. Because reproductive success in Tawny Owls is clearly food limited (Southern 1970, Hirons 1985, Jedrzejewski et al. 1996, Appleby et al. 1997, Sasvári & Nishiumi 2005, Sunde 2005, this study), the apparent differences in sex allocation patterns are likely to be related to different food regimes. The Danish population probably deviates from the other two Tawny Owl populations, because the main energetic bottleneck for reproduction occurs before hatching. This is indicated by the high variability in breeding frequency, clutch size and hatching dates across years, whereas nestling mortality due to starvation was low. Differential starvation risks in poor breeding seasons across study areas also seem to exist after fledging, as 5% of all young in the Danish population starved between fledging and independence in years of poor food conditions (Sunde 2005), as opposed to more than 50% in the Northumberland population (Coles & Petty 1997). Specific differences in the timing of resource constraints relative to the timing of breeding may imply that females in the Danish population that were able to lay a clutch in a given year could predict improving food conditions up to hatching of their clutches rather precisely, as was indicated by increasing trap indices of small rodents in both good and bad years (Bølstad 2001). Compared with the two previous studies of sex allocation in Tawny Owls, all broods in the Danish population therefore seemed to experience good or moderately good food conditions from hatching onwards. This means that simple phenotypic plasticity may explain the discrepancy in sex allocation pattern between this and previous studies at and within the brood level. The apparent lack of conditionally induced variation in sex allocation patterns at (and within) brood level in the Danish population may therefore reflect absence of sufficiently strong variation in food conditions post-hatching to favour the appearance of conditional sex allocation strategies across broods. However, phenotypic plasticity is unlikely as an explanation for the higher overall proportion of males in the Danish population. If the Danish Tawny Owls were genetically predisposed to respond to changes in food conditions in the same way as females in the two other study areas, they should have produced female-biased broods instead of the male-biased broods that were observed throughout the four years. We thus hypothesize that there may be genuine genetic differences in breeding and sex allocation strategies across populations as a set of regional adaptations to expected food conditions throughout the year. If we are right in our interpretation, Tawny Owl populations characterized by high inter-annual variation in laying frequency but low inter-annual variation in starvation-related post-hatching mortality should produce male-biased broods not conditional on the feeding situation. This appears to be the ecological situation in deciduous lowland forests in central Europe, where rodent populations are controlled by autumnal mast crops but vary predictably within years, e.g. Wytham wood, Oxford, UK (Southern 1970, Hirons 1985) or Bialowieza forest, Poland (Jedrzejewski et al. 1996). Conversely, populations characterized by high variability in starvationrelated offspring mortality (e.g. due to cyclical prey populations or variable weather conditions across years) should express variation in brood sex ratios conditional on the actual foraging situation. We offer this hypothesis for future testing. Ringing was licensed (personal licence A-543 to P.S. and B-573 to K.B.D.) by the Ringing Centre, Zoological Museum (ZM) of Copenhagen. Blood sampling for DNA analyses was approved by the Danish council for Animal Experiments to ZM ( : licence no / 00-2) and P.S. ( : licence number 2002/ ). We thank the Frederiksborg State Forest Commissions, the Jarl Foundation and private landowners for access to their properties. Bent Jensen assisted during the fieldwork. The study was supported by grants from the University of Copenhagen (to K.B.D.), the Danish Natural Science Research Council (to J.J.B.) and the Carlsberg foundation (to P.S.). Gary Bortolotti and Tim Hipkiss commented on a previous version of the manuscript.

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