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

Abstract

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.

Population dynamics in a cyclic environment: consequences of cyclic food abundance on tawny owl reproduction and survival

“…Thus, our result differs from studies of tawny owls in temperate regions without multi-annual fluctuations in the abundance of their main prey. In particular, territory numbers in temperate regions are stable between years (Southern 1970;Hirons 1985;Jedrzejewski et al 1996;Sunde & Bølstad 2004;Desfor, Boomsma & Sunde 2007). We find that the variation in territory number between vole cycle phases is not driven by extensive mortality after the decrease phase as in the closely related Ural owl living in the same environment (Brommer et al 1998(Brommer et al , 2002, because tawny owl survival is fairly stable between vole phases.…”

Brood sex ratio varies with diet composition in a generalist raptor

“…When conducting family‐level analyses, I tried to overcome some long‐standing, and largely overlooked, conceptual and methodological problems in sex‐ratio research. Several earlier studies on raptors have investigated correlations between brood sex ratios and food resources, but their results were often difficult to interpret, because: (1) they measured environmental prey abundance, not realized resource use; and (2) they integrated prey abundance over large areas, typically the entire study plot (Bednarz & Hayden, 1991; Wiebe & Bortolotti, 1992; Dzus, Bortolotti & Gerrard, 1996; Leroux & Bretagnolle, 1996; Appleby et al ., 1997; Korpimäki et al ., 2000; Byholm et al ., 2002a; Hipkiss & Hörnfeldt, 2004; Laaksonen, Lyytinen & Korpimäki, 2005; Millon & Bretagnolle, 2005; Desfor, Boomsma & Sunde, 2007). While some of these studies were concerned with population‐level patterns, others aimed to examine individual‐level responses, violating critical theoretical assumptions.…”

Secondary sex ratio of nestlings of the blackbird (Turdus merula)

“…al. 1997), but the population in Denmark produces more male chicks in clutches produced during years with high rodent numbers - (Desfor et. al.…”