Small and isolated populations are usually assumed to be at a high risk of extinction due to environmental or demographic stochasticity, genetic problems, or too little immigration. In birds, natal dispersal is usually female‐biased, but the consequences of such a pattern on vulnerability to extinction of isolated populations has not received much attention before. In this paper I derive predictions as to how female‐biased natal dispersal may differentially affect the extinction risk of populations and species with contrasting distributions, migratory behaviours, life histories and mating systems. Female‐biased dispersal will lead to male‐biased sex ratios in small, isolated or fragmented populations, in particular because recent research has shown that females often have a limited ability to search for mates and may therefore effectively be lost from the breeding population if they disperse into areas empty of conspecifics. I reviewed published studies on birds and found that a high proportion of unpaired males is common in isolated populations or populations in small habitat fragments. Dispersal of females may therefore increase the vulnerability to extinction of small or isolated populations, or populations at the periphery of a species’ distribution range. I also predict that vulnerability to extinction should be greater for migratory than for resident species and greater for short‐lived than for long‐lived species because of differences in the time available for females to locate unpaired males. Further, extinction risk may also be greater for birds than for mammals due to differences in which sex disperses and patterns of parental care. Finally, mating system will also affect vulnerability to extinction when natal dispersal leads to biased sex ratios. I review available evidence for these predictions (e.g. songbird declines in North America) and discuss implications for conservation.
The mate sampling behaviour and mate choice of 125 individually marked female pied flycatchers, Ficedula hypoleuca, was recorded with video cameras. Females visited on average 3.1 males within a period of less than 1 day. Females had independent preferences for unmated males, brightly coloured males and males with nestboxes that had small entrance holes; 77-86% of the females chose a male that was the best one among the males sampled in relation to at least one of these factors. When having a choice only on mating status 91 % of the females made a correct choice; corresponding values for plumage colour and nest site quality were 64% and 73%, respectively. Females made 'mistakes' more often when differences between males were small, at least regarding plumage colour. When given simultaneous choices on two cues, females gave priority to the cue with the largest differences between males. Females returned to some males before rejecting them and these males tended to have brighter plumage or better nestboxes than males that were visited only once. The final choice of mate was not related to the order in the sequence of males visited, suggesting that many females used a pool-comparison strategy. However, about one third of the females visited only one male, and one third sampled males in a way conforming to threshold or sequential comparison strategies as well as a pool-comparison strategy. The temporal pattern of female visits suggested that they either sampled males at once and then settled, or that they visited one male repeatedly and made occasional visits to other males, often after a long period of residency. Twenty-eight females (22%) made visits to other males after they had settled
(i.e.
started nest building) with one male, and this resulted in mate switching in seven cases (6% of all females). These results show that females compare and choose mates on the basis of at least three different cues, and that most females are able to pick out the best or one of the best males among those sampled. However, females sample few males, probably because of competition between females for a mate, so that they must sometimes accept dull or mated males, or males with poor-quality nest sites. Limited choice reduces the strength of sexual selection even when female preferences are strong. This may help explain why plumage colour variation and polygyny exist in the pied flycatcher.
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