Breeding

Oilbirds nests are built on ledges 10-20 m above cave floor (Snow 1961, Bosque 2002). Consists of a bulky nest: short, truncated cone topped by a shallow depression, about 40 cm in diameter, made of regurgitated fruits and seeds mixed with the bird’s own excrement, and grows higher each year as more material is added (Snow 1961). This regurgitated material is compacted into a firm paste. Nests are often very close together and are always inside caves (Hilty 2003).

2-4 white eggs are laid at intervals of 2-6 days (Snow 1961, Bosque 2002). Incubation, brooding and feeding of the young are shared within both parents (Snow 1961, Bosque 2002). Incubation periods last 32-45 days and begin when the first egg is laid (Snow 1961, Bosque 2002). Adults must visit nests often to prevent chilling of eggs and young nestlings due to low temperatures in caves (Wittenberger and Tilson 1980).

Newly hatched birds (12-15.5 g) bear an egg tooth, are pinkish, almost completely naked and their eyes are closed. They grow very slowly, possibly due to the low protein content of the diet, leaving the nest when they are 110-120 days old. By day 70, nestlings have an appearance similar to that of adults, except that the tail and wings are shorter (Snow 1961, Bosque 2002).

Breeding is highly seasonal, mainly coinciding with the rainy season. However breeding varies among populations. In the Caripe Cave in Venezuela, most egg laying occurs in the second half of April and May which coincides with the fruiting peak of Lauraceae (Bosque 2002). Nearby Trinidad, breeding is less seasonal and birds are found in different breeding stages at any time of year (Snow 1961).