Experimental Brood Manipulations and the Monoparental Threshold in Snail Kites

Abstract

The effect of brood size (the expected benefit of reproduction) on the decision of a snail kite parent to desert its mate or not was examined by manipulating brood size (normally one to three young). Mate desertion occurred at about two-thirds of the nests monitored, with females deserting twice as often as males. Fledging success was sharply reduced for enlarged broods: kite parents had difficulty raising three young and were unable to raise four young. Low food-delivery rates, coupled with partial brood reductions of enlarged broods, indicated that starvation was the most likely cause of most offspring mortality. Desertion occurred at all experimental nests fledging one young, at half of the nests fledging two young, and at none of the few nests that were able to fledge three young. Similar patterns of fledging success and desertion were found for unmanipulated nests, except that parents attending unmanipulated nests were unable to fledge three young successfully. The occurrence of mate desertion was no...