A breeding ecology of the endangered palila (Psittirostra bailleui) / Breeding ecology of Palila and ʻAmakihi parasites on Mauna Kea, Hawai'i

Abstract

The behavioral ecology and breeding biology of the endangered Palila (Psittirostra bailleui) was studied from 1971 to 1975. The most intensive breeding occurred from June to August, and coincided with peak production of mamane (Sophora chrysophylla) seeds, the bird's major food source. The Palila was able to make adjustments in its breeding to compensate for yearly differentiation in the timing and abundance of this food supply. Sexual chasing and courtship feeding were the most frequently encountered pre-nesting behaviors. Territory was a mate-defended area, which later in the nesting sequence was confined to the nest site. A total of 26 nests was found; most were placed on larger branches of mamane trees. Nest construction occurred primarily in the morning hours and lasted up to 20 days. Both sexes took part in nest construction, albeit the male role was minimal. Unless the nest was placed in the terminal fork of a tree, it usually contained a large stick base. The modal clutch size was two; eggs were laid early in the morning and in all cases one per day. Incubation sometimes began with the first egg and lasted 15 to 16 days. Only the female incubated, and she covered the eggs for about 75% of the daylight hours and throughout the night. Egg hatching was asynchronous, with the first young emerging early in the morning and the second not until later that same day. Only the female brooded, and the rate declined until day 15 when essentially it stopped. Both parents fed the young by regurgitation, and the number of feedings per hour decreased slightly over the nestling period. It is thought that insects and finely masticated plant material formed the bulk of the nestling diet until about day 5 when mamane seeds became important. Helpers were found at one nest. Young developed slowly and did not leave the nest until 21 to 27 days old. It is believed that these prolonged nestling periods were able to evolve because of the (former) absence of ground predators. After fledging, young remained with their parents for at least 30 days. Productivity was regulated by small clutch size, low population numbers, and by the length of an individual nesting sequence (in that a pair could potentially raise only one brood each year). The primary reason for the endangered status of this bird appears to be the effect of habit at alteration upon a specialist, coupled with the fact that the small effective breeding population and low dispersability of the species may have resulted in decreased genetic fitness.