Extra-pair paternity (EPP), where offspring are sired by a male other than the social male, varies enormously both within and among species. Trying to explain this variation has proved difficult because the majority of the interspecific variation is phylogenetically based. Ideally, variation in EPP should be investigated in closely related species, but clades with sufficient variation are rare. We present a comprehensive multifactorial test to explain variation in EPP among individuals in 20 populations of nine species over 89 years from a single bird family (Maluridae). Females had higher EPP in the presence of more helpers, more neighbours or if paired incestuously. Furthermore, higher EPP occurred in years with many incestuous pairs, populations with many helpers and species with high male density or in which males provide less care. Altogether, these variables accounted for 48% of the total and 89% of the interspecific and interpopulation variation in EPP. These findings indicate why consistent patterns in EPP have been so challenging to detect and suggest that a single predictor is unlikely to account for the enormous variation in EPP across levels of analysis. Nevertheless, it also shows that existing hypotheses can explain the variation in EPP well and that the density of males in particular is a good predictor to explain variation in EPP among species when a large part of the confounding effect of phylogeny is excluded.
Parasitic cuckoos of the genus Clamator and parasitic cowbirds Molothrus spp. lay eggs that appear to be stronger than those of their hosts. Similar differences in eggshell strength between the eggs of the Cuculus/Cacomantis/Chrysococcyx cuckoos and their hosts are not apparent. We suggest that the advantage of a strong eggshell in the Clamator and Molothrus species is to protect the parasite's egg from damage if the nest is multiply parasitized. This suggestion relies on the observation that Clamator and Molothrus species commonly lay more than one egg per nest and often remove or damage eggs in the process.
The blue-breasted fairy-wren, Malurus pulcherrimus, is a small, permanently sedentary, socially monogamous, co-operatively breeding, long-lived Australian passerine, whose core distribution is contained within the Western Australian wheatbelt. Little is known of their dispersal capabilities and population dynamics, knowledge of which are urgently needed as their preferred habitat becomes increasingly fragmented and degraded, due to clearing for agriculture.
From 1993 to 1998, we studied a colour-banded population of blue-breasted fairy-wrens living in 13 small, 3 medium-sized and one large habitat remnant in an agricultural landscape at Wyalkatchem. Wrens in small remnants were more likely to disperse between remnants than were those in larger remnants. Females were likely to travel further and were more likely to disperse between remnants than were males. Dispersing females in the largest remnant moved a median straight-line distance of 400 m compared with 3 km for females in the smaller remnants. The furthest recorded movement was 9 km (straight-line distance) or about 14 km by the nearest corridor route. Females living in fragmented habitat may have to travel many times the distances travelled by females in unfragmented habitat.
Demographic modelling shows that, over an average run of seasons, the wrens in the study population will produce sufficient yearlings to fill all breeding vacancies, provided none are lost during dispersal. However, in our study landscape, an estimated 11% of dispersers were lost to the population (i.e. either dispersed outside the study area or died during dispersal). We show that the remnants in the study landscape can be divided into neighbourhoods based on their relative connectivity. About 14% more potential dispersers were lost from the poorly-connected areas than from a core 'well connected' neighbourhood. This difference would be sufficient to cause population decline in a poorly connected neighbourhood. We conclude that, for species with poor dispersal capacity living in fragmented habitat, failure to maintain an adequate corridor network could be one of the most important factors contributing to a species' decline.
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