A male house sparrow feeding a chick in a bird box © Tony Margiocchi / Barcroft Media / Getty
Yes – the black ‘bibs’ sported by the males do vary in size. Researchers have found that bib size is linked to social status, age and sexual behaviour. Males with larger bibs obtain mates earlier in the breeding season, copulate more frequently and engage in a greater number of extra-pair copulations (matings with females that are not their partner). Their breeding territories have a bigger proportion of breeding sites, and they participate more often in communal displays.
Since female house sparrows actively select males with larger bibs, size does seem to provide an honest signal of quality, though there is still uncertainty over which aspects exactly. Investing in the black (melanin-based) plumage carries a cost for the male, so males that can afford to cover this outgoing are perhaps signalling that they are more fertile or will father better quality offspring.
Smaller-bibbed males end up occupying poorer quality territories and may be forced to feed in riskier locations than their larger-bibbed rivals, who dominate the best sites.
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