Experimental Brood Manipulations and the Monoparental Threshold in Snail Kites

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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 not related to clutch size, Julian date of nest initiation, or whether nests were colonial or solitary. Deserted nests fledging two yound did not have higher food-delivery rates than undeserted counterparts. Desertion tended to occur earlier in one-young broods than in two-young broods after the effects of brood reduction were considered. Nests begun earlier in the breeding season were deserted earlier than those begun later; differences also existed between years. These results demonstrate how brood size can act as a proximate factor controlling the mating system through the demands of parental care. A monoparental threshold, as reflected by both the occurrence and the timing of desertion, is based on the relationship between food demand (by the brood) and supply, and it can be crossed if a single parent is capable of caring for the offspring alone. That snail kite parents of experimentally enlarged broods could not fledge four young, had difficulty fledging three young, and did not increase food-delivery rates proportionally with brood size supports Lack''s hypothesis that food limits clutch size in tropical birds.