Bushtit Babies

I came across my first family group of Bushtits of the year, today.  The group was comprised of what looked like three adults, which were probably the mated pair and one nest helper, and four young birds.  Bushtits are one of a number of species that include nest helpers in their reproductive strategy.  These helpers are usually young from the previous year that return to aid their parents.  Young birds that fledge from a nest early in the year also sometimes stay to act as helpers as their parents raise a second brood of young.  Since this family group that I found is so early I would not be surprised if the adults attempted a second brood.

A number of ideas about why young birds help raise their siblings are floating around.  One is that the young birds are getting valuable experience in raising babies, so that they will have better success when they do eventually go off by themselves.  Another idea is that the baby birds grow faster, and so fledge earlier, when there are more helpers to feed them.  A related idea has to do with what is called inclusive fitness.  This hypothesis says that the more individuals that carry a copy of a particular gene, the better, so helping relatives survive is actually a good strategy for preserving your own genes.  This is especially true if the individuals in question are close relatives, such as siblings, because there is a lot of shared genetic information.  A further idea is that good nesting territories are few and far between, so the helpers return to their parents territory and stay around in the hope of inheriting that nesting territory if and when their parents die or breed elsewhere.  This seems particularly possible in cavity nesting birds for which cavities have been shown to be a limiting factor in reproduction.  All these hypotheses seem reasonable, and they are not necessarily mutually exclusive, but few have been really well examined!  This seems to be an area that is ripe for some observations and experiments that might yield really interesting results.

So, who wants to put up a bunch of bird boxes and see what happens?

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