Breeding behaviour, clutch size and egg dimensions of Neospiza buntings at Inaccessible Island, Tristan da Cunha

Abstract

Inaccessible Island, Tristan da Cunha, supports two colour morphs of Tristan Buntings, Nesospiza (a.) acunhae, Wilkins' Buntings, N. (wilkinsi) dunnei, and interspecific hybrids between Tristan and Wilkins' Buntings. Breeding takes place in the austral summer (November-February). The onset of breeding (mid-November to early December) was similar in 1989/90 and 1999/2000, but may commence earlier in some years. The larger and rarer Wilkins' Buntings started breeding 1–2 weeks earlier than Tristan Buntings. Basic breeding biology was similar among all populations at Inaccessible Island. Females undertook all nest construction, building relatively large, open cup nests on the ground or up to 1.2 m up in dense tussock gross, Spartina arundinacea. Clutch size was two, or occasionally one, eggs, and did not differ between taxa (mean 1.81 ± 0.40). For two-egg clutches, egg size differed by 7–11% on average. In all cases where laying sequence was known, second-laid eggs were smaller than first-laid eggs, and larger eggs almost invariably hatched first. Egg size was related to body size, and upland Tristan Buntings laid significantly larger eggs than lowland Tristan Buntings, Nesospiza eggs are similar in size to those of other finch-like members of the tribe Thraupini, but within the genus egg size increases more slowly relative to body size than expected. This pattern is shored by Darwin's Finches; we infer that evolution in body size has outpaced evolution in egg size. Wilkins' Bunting eggs were more elongate than those of Tristan Buntings or hybrids, which mirrors a trend for egg elongation to increase with body size among finches. Incubation lasted 17–18 d and was solely by the female. Chicks remained in the nest for 18–22 d, averaging slightly longer for larger taxa. After fledging, juveniles were fed by the parents for up to 11 weeks, during which time they remained in the natal territory. Brood division occurred among pairs that raised two chicks, with a tendency for male parents to care for female offspring and vice versa. The long period of post-fledging parental care prevented pairs raising more than one brood each year, although repeat clutches were common following breeding failure.