Cowbirds are brood parasites, meaning that they require the parental care of other species to rear their own offspring. This is a behaviour common in birds, with the cuckoo a well known example. The benefits of brood parasitism are obvious – brood parasites can have more offspring as they provide little parental care.
However, it is less clear why other species tolerate this obvious cheating. Cowbirds eggs look clearly different to their hosts’ eggs. Hosts of cuckoo eggs have evolved to eject unfamiliar looking eggs, but this is not the case for cowbird hosts. So what causes this difference?
Retaliatory behaviours similar to that of the mafia have been reported in brood parasites. This response involves the destruction of a host’s nest after a parasite’s egg is ejected. A study testing this hypothesis, in Illinois, USA, looked at warbler nests, a common target for the cowbird. They used manipulative techniques to restrict cowbird access to some warbler nests including:
- Creating nest boxes with an entrance large enough for a warbler, but too small for the parasitic cowbird. This meant that they could observe control nests, where access after laying the egg is limited.
- Removing cowbird eggs from warbler nests by hand, imitating a host’s decision to eject an egg, allowing the scientists to observe the cowbird reaction
The study found that half of all nests that had rejected the parasite’s eggs had been destroyed. This compared to almost no cases when the cowbirds eggs were accepted by the host warblers. The control groups confirmed the suspicions that cowbird mafia-style behaviour are the cause of these nest destructions. Nests that prevented cowbird access were not destroyed, even in nests that had ejected a cowbird egg.
The study also looked at the evolutionary mathematics of complying with the cowbird mafia:
- Nests that rejected cowbird eggs, and were later destroyed in retaliation, produced the least offspring (averaging just 1 per nest)
- Nests where cowbird eggs were accepted had significantly more offspring (on average 3)
So the study concluded that ejecting behaviours in warblers are unlikely to evolve due to the retaliatory ‘mafia-like’ behaviour of the cowbird. This can be best understood by thinking of these complex behaviours as single gene traits. A gene for rejecting cowbird eggs would not persist, as those with the gene would have less offspring than those without it. Therefore, the gene for complying with the mafia spreads.
For more summaries of published scientific papers read my other Research Reviews.