BREEDING BIOLOGY OF THE SCARLET IBIS ON CAJUAL ISLAND, NORTHERN BRAZIL

The reproductive biology of the Scarlet Ibis (Eudocimus ruber) and White Ibis (E. albus) is well known from sites in North America, the Caribbean, and the Venezuelan Ilanos, but few data are available on the status or breeding patterns of the populations that inhabit South American mangroves. From 1992-1997, we monitored the status of a colony of E. ruber in an area of mangrove in northern Brazil. The breeding population numbered 2000-3400 pairs. We did not find any significant population fluctuations between years; apparently, most birds attempted to breed each year, in contrast with freshwater-breeding populations. Clutch size was 1.97 +/- 0.17, lower than at more northern saltwater sites. The low variability of clutch size was surprising. Whole nest failure from catastrophic events such as storms, human predation, and nest desertion was responsible for most egg and nestling mortality. Partial mortality was rare. Singleton eggs and nestlings were frequently abandoned by adults following the loss of an egg or nestling. We conclude that this population pursued a strategy of brood survival rather than brood reduction which is usually observed in Eudocimus. This may have resulted from stable resource availability at this site. Given that northern populations often do not breed in unfavorable years, selection appears to have favored adult survival over reproductive capacity in this species. Human disturbance was intense, and it can threaten ibis populations both through direct predation and induction of nest desertion.

CARLOS MARTINEZ, ANTONIO AUGUSTO and FERREIRA RODRIGUES, J. Field Ornithol., 70(4):558-566