Frequency-dependent selection generally leads to an equal expenditure by parents on offspring of both sexes

equal expenditure by parents on offspring of both sexes (Fisher 1930). When the cost of rearing the sexes differs, the offspring of more beneficial sex will be produced in a higher proportion. However, a growing number of studies report deviations from this prediction. Biases in sex ratio depend on environmental conditions, particularly food availability (Torres & Drummond 1999), and as these conditions are not stable, the sex ratio may change seasonally and annually (Edwards et al. 1988, Korpimäki et al. 2000). The resulting biases in sex ratios may be also an important target for conservation planning, whereas the risk of unbalanced sex ratio could occur in small populations of endangered species (Clout et al. 2002). In sexually dimorphic species, the larger sex is more costly to rear because of higher energy requirements (Riedstra et al. 1998). In raptors and owls, females are larger and, consequently, the sex ratio of nestlings is male-biased in years of food-scarcity (Korpimäki et al. 2000) or in prey-poor territories (Appleby et al. 1997). However, even among raptor species, different studies have given contradictory results (see Torres & Drummond 1999 for references). I investigated nestling sex ratio both at the population and family (nest) levels in Lesser Spotted Eagle Aquila pomarina. This long-lived raptor has an unfavourable conservation status in Europe (Tucker & Heath 1994), and it almost always rears only one nestling per brood. Although two eaglets hatch, one of them usually dies during its first weeks of life (Meyburg 2001). I compared nestling sex ratios and their deviation from unity during more and less favourable breeding years, and examined the ability of adults to vary sex of their offspring between years. The study was performed in Estonia (58–59°N, 24–27°W) where the productivity of the Lesser Spotted Eagle is positively associated with three-year cycles in the abundance of field voles Microtus sp. , their main prey (Lõhmus 1999, Väli 2003). The study included two vole-peaks (1999 and 2002), and two vole-poor years in between. Eagle nests were checked in late July when nestlings were 40–50 days old and the broods had already been reduced by siblicide. The nestlings were sexed using the sex-specific chromo-helicase-DNA-binding (CHD) gene. DNA was extracted from a blood sample collected by brachial vein puncture. A part of the CHD gene was amplified with primers P2 and P8 according to Griffiths et al. (1998). The products from the first four randomly selected blood samples were sequenced. Within these, both sexes were represented as confirmed by the comparison with sequences from other species (Griffiths et al. 1998). Two gene forms showed a different cutting pattern of restriction enzyme VspI (Fermentas). The product of CHD-W contained two restriction sites whereas that from CHD-Z only one, which gave three DNA bands for females and two bands for males – a clearly visible difference on 2% agarose gel. Thus, electrophoresis following VspI restriction of the amplification product from the CHD gene was routinely used for sexing the nestlings. Altogether, 118 successful broods were studied during the four years. A brood of two fledglings was observed only once; the older nestling (by wing-length) was a male and the younger a female. In all other cases only one nestling fledged. Because there are two poor years per one good one in a vole cycle, I used the weighted average of poor and good years to test for a biased sex ratio. This average sex ratio of nestlings did not differ from parity, but the number of females exceeded the number of males in good years, and vice versa in poor Bird Study (2004) 51, 189–191