Oecologia (2003) 137:140–147 DOI 10.1007/s00442-003-1317-1 BEHAVIOURAL ECOLOGY Jon E. Brommer · Patrik Karell · Tuomo Pihlaja · Jodie N. Painter · Craig R. Primmer · Hannu Pietiäinen Ural owl sex allocation and parental investment under poor food conditions Received: 17 March 2003 / Accepted: 19 May 2003 / Published online: 26 June 2003  Springer-Verlag 2003 Abstract Parents are expected to overproduce the less costly sex under poor food conditions. The previously regular 3-year cycle in the abundance of voles, the main prey of the Ural owl, Strix uralensis, temporarily disappeared in 1999–2001. We studied Ural owls’ parental feeding investment and sex allocation during these poor-quality years. We sexed hatchlings and embryos in unhatched eggs of all 131 broods produced during these years. Population wide, the owls produced significantly more males (56%). The parental food investment in the brood was estimated by sorting out the prey remains in the bottom of nest boxes. Food delivered to 83 broods without chick mortality showed no clear sex-specific investment. Nestling mortality was equal in both sexes. Thus, evidence for an investment- driven sex allocation is weak. Neither laying date, brood size nor the female’s condition correlated with offspring sex ratios. In these poor years, parents provided less food per chick and the fledging weight of daughters was reduced more than the weight of sons compared with years of high food abundance (1983 and 1986). We discuss, in relation to published studies, the possibility of a sex-allocation scenario where, under poor food condi- tions, a daughter’s long-term fitness is reduced more than a son’s. Keywords Life history · Sexual dimorphy · Sex ratio · Vole cycle Introduction There is increasing evidence of facultative sex allocation at the family level in vertebrates, although this phenom- enon has traditionally been considered rare (Charnov 1982; Clutton-Brock 1986; Clutton-Brock and Iason 1986). Most often, sex bias is environmentally induced. Examples of environmental factors affecting sex ratios include territory quality (Seychelles warbler, Bebornis sechellensis: Komdeur et al. 1997; tawny owl, Strix aluco: Appleby et al. 1997), paternal attractiveness or quality (collared flycatcher, Ficedula albicollis: Ellegren et al. 1996; blue tit, Parus caeruleus: Sheldon et al. 1999) or social environment (spotted hyena, Crocuta crocuta: Holekamp and Smale 1995). In general, parents are expected to overproduce the sex which has the highest (reproductive) value (Fisher 1958; Trivers and Willard 1973; Leimar 1996), but deviations from parity in diploid vertebrates are usually less pronounced than in, for example, haplo-diploid insects (Charnov 1982). There are a variety of sex-allocation theories and hypotheses which can, to some extent, be applied to vertebrates (Pen and Weissing 2002). These can be broadly categorised into two approaches, one concerning population-wide sex allocations and one concerning family-level sex allocation. Fisher (1958) argued that individuals are selected to produce offspring at an equal sex ratio, because of frequency-dependent selection. If the population-wide sex ratio differs from parity, the (repro- ductive) value of the rare sex would be higher and parents would be selected to produce more of the rare sex until parity is restored. However, if the parental investment differs between sons and daughters, the optimal popula- tion-wide sex ratio should be inversely related to the investment ratio (Fisher 1958). Fisher’s sex allocation mechanism has been invoked as an explanation for sex allocation processes on an ecological time scale (e.g. Bensch et al. 1999; Ranta et al. 2000). Whereas Fisher’s sex-ratio mechanism operates at the population level, Trivers and Willard (1973) considered sex-ratio adjustment at the family level. Their original J. E. Brommer ( ) ) · P. Karell · T. Pihlaja · J. N. Painter · H. Pietiäinen Department of Ecology &and Systematics, Bird Ecology Unit, University of Helsinki, Viikinkaari 1, P.O. Box 65, 00014 Helsinki, Finland e-mail: jon.brommer@helsinki.fi Tel.: +358-9-19157684 Fax: +358-9-19158701 C. R. Primmer Department of Ecology and Systematics, University of Helsinki, Viikinkaari 1, P.O. Box 65, 00014 Helsinki, Finland