How do Black-winged Kites know when to breed?
Several features of Black-winged Kite come from their specialisation on rodent prey which typically fluctuate widely in abundance from season to season, and from place to place. One feature is the ability to breed more than once, and at any time of the year. Another is to move widely and opportunistically to track rodent prey populations. And yet one more is their apparent ability to anticipate when prey will be available in sufficient abundance to feed their chicks.
Kites normally eat 1 or 2 mice each per day out of the breeding season, whereas during the peak delivery period in the nestling and post-nestling period, adults need to catch about 5 to 8 mice per day! 'Knowing' when they could hunt so many mice would be of obvious value for successful breeding. But how might kites anticipate future increases in rodent numbers that might happen at any time of the year? What could stimulate breeding?
Conventional reproductive stimuli such as day length, food supply, temperature and circannual rhythms would not work. Another hypothesis suggested by John Mendelsohn of the Ongava Research Centre is that the kites' reproductive activity might be triggered by levels of reproductive hormones in their prey. Thus, if the kites ingest large amounts of testosterone- or oestrogen-like hormones that would signal that there were (a) lots of mice around and (b) lots of them would themselves be breeding - an efficient cue that an abundance of mice could be available to feed kite chicks 2 to 4 months hence!
Support for this being a credible idea actually comes from other work on rodents that are stimulated to breed by the amounts of plant oestrogens they get in their food. And fleas begin to breed when their rabbit hosts have high levels of reproductive hormones, which signal the future presence of baby rabbits on which baby fleas can disperse. To our knowledge, this idea and the efficient mechanism it implies for kites and other raptors (certainly the two owls) has not been tested. Imagine a food chain tempered by reproductive hormones: from those in grass, to rodents to raptors and doubtless parasites of the rodents. Who else might benefit from monitoring hormone levels? Those are some questions for an eager young biologist to get his or her teeth into.