EXPERIMENTAL BROOD MANIPULATIONS AND THE MONOPARENTAL THRESHOLD IN SNAIL KITES

Mate desertion in Rostrhamus sociabilis occurred at c 2/3 of the nests monitored in Venezuela, with females deserting twice as often as males. Fledging success was sharply reduced for enlarged broods: kite parents had difficulty raising 3 young and were unable to raise 4 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 1 young, at half of nests fledging 2 young, and at none of the few nests that were able to fledge 3 young. Occurrence of mate desertion was not related to clutch size. Julian date of nest initiation, or whether nests were colonial or solitary. Desertion tended to occur earlier in 1-young broods than in 2-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. Results demonstrate how brood size can act as a proximate factor controlling the mating system through the demands of parental care. A monoparental threshold, reflected by both the occurrence and 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. -from Author