Embryonic Mortality and Sex Ratios in the Tree Sparrow

Tree sparrows (Passer montanus) have been studied in two areas in Sweden since 1997. At both sites, tree sparrow eggs had remarkably low hatching success. On average only 60% of the eggs hatched. Analyses have shown that this was caused by embryonic mortality, which was highly sex biased. About 70 % of the dead embryos were males, while about 65 % of all fledged nestlings were females. Impaired hatching success here related to two factors. Hatching success was lower for pairs with a male in poor body condition, and it was lower in areas with a high local population density.A sex bias in the mortality early in life has been demonstrated in several species. Since the competitive ability of males is determined by conditions early in life, parents with poor provisioning capacity should prefer to produce female offspring in broods reared under poor conditions. The body condition of a tree sparrow during the nestling stage was well correlated to the condition as an adult, and pairs in which the male parent was in poor condition produced chicks in poor condition. Since the breeding success of a pair depended more on the condition of the male, females appear less affected by conditions early in life. Parents with poor provisioning capacity appear to bias offspring survival towards females, and a difference in the early susceptibility may be adaptive…


Material and Methods
Study species
Study areas
Field work methods
Sex determinations
Fertility analysis (I)
Endocrine disrupting substances (II)
Adaptive sex ratios (III)
Nest site choice and population density (IV)
Parasites (V)
Results and discussion
Breeding success and sex ratios (I)
Effects of estrogenic substances on sex ratio and survival (II)
An adaptive view on sex biased mortality (III)
Nest site preferences (IV)
Effects of parasites (V)
(summary in Swedish)

Author: Svensson, Magnus

Source: Uppsala University Library

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