Male birdsong is known to perform two key functions: attracting a female and repelling rival males. In kalahari white-browed sparrow weavers (Plocepasser mahali), the dominant males monopolise access to breeding opportunities, both within- and extra-pair mating all goes to the dominant males: subordinate males don't get a look-in. Dominance tenures are also very long, therefore in order to breed, subordinate males must acquire a dominant position.
Dominant male white-browed sparrow weavers sing pretty much every day at dawn during the breeding season. Their territories are very densely-packed so a group's roosting tree might be just 100m from their neighbour. Waiting in the darkness at dawn, it is like sparrow weaver surround-sound as all the males in the area one-by-one begin their dawn song performance. You really get a feel for what's going on in the neighbourhood. So it occurred to me, perhaps subordinate males monitor the dominant males in the neighbourhood. This might give subordinates a good idea about their future prospects of promotion - if a dominant male's song performance is poor, then perhaps he is on his way out or is temporarily weakened and now is a good time to attempt a take-over.
So the first step was to ask: is dominant male dawn song performance a good indicator of current condition? I gave dominant males a small immune-challenge to see whether this would affect their dawn song performance. Would they stop altogether or would they sing less than the previous day? And across males, would the strength of their immune response be related to their dawn song performance?
The answer was yes... dominant males sang less the day after an immune challenge, but interestingly, they all still sang. The more a male sang on the day before the immune-challenge, the stronger was his immune-response to that challenge, suggesting that, at least in this species, dawn solo song might tell other birds something about a given dominant male's current immune status. The open access paper is available here if you want to know more.
The next step would be to find out whether subordinate males utilise this information during dispersal, in order to maximise their chances of winning the jackpot and becoming the dominant male in a group.
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