The breeding biology of two populations of the white-rumped swiftlet (Aerodramus spodiopygius assimilis) in Fiji and (Aerodramus spodiopygius chillagoensis) in Queensland, with special reference to factors that regulate clutch size in birds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University

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White-rumped Swiftlets Aerodramus spodiopygius (Apodidae) build nests of vegetable material and cement (from their saliva) in the dark sections of caves at Chillagoe in Queensland, Australia, and in Fiji. Fijian colonies average 1,762 nests while the colonies at Chillagoe contained an average of 77 nests. Breeding takes place between September and March in Fiji, and from October to March at Chillagoe. There is no sexual dimorphism and both sexes share in incubation and the feeding of nestlings. At Chillagoe the clutch is one egg whereas in Fiji it is two eggs laid three to five days apart. At Chillagoe incubation took 27. 8 days in the poor year and 26.6 days in the good year. In Fiji incubation averaged 23 days and 58% of eggs hatched compared to 64% of eggs at Chillagoe. The Fijian birds successfully fledged an average of 92%, a breeding success of 53% or 1.1 young fledged per breeding pair. From the two single-chick broods the birds at Chillagoe fledged 69%. a breeding success of 44% or 0.9 young fledged per pair in the good breeding season. In the poor year at Chillagoe hatching success was 60%. fledging success was 50%, reducing breeding success to 30%. At Chillagoe the fledging period was increased from 46.9 days in the good year to 49.8 days in the poor year. At both locations most chick mortality resulted from chicks falling from their nests. Lost eggs or chicks were normally replaced by eight to fourteen days. Chicks in Fiji were fed an average of 2.2 times a day, whereas those at Chillagoe were fed an average of 5.2 times a day in the good season and 3.0 times a day in the poor season. Placing the data for this species with those for other species of apodids shows a positive correlation between egg size and adult size and a negative correlation between feeding frequency and the nestling period. Producing a third egg would not benefit the Fijian Swiftlet, which could not hatch significantly more eggs when given a third egg and could not fledge significantly more chicks when given three chicks instead of their normal brood of two. Fijian birds fed the artificially enlarged broods more frequently than normal sized broods, but neither the number of feeds per chick nor the number of chicks fledged in the larger broods was increased. Parents are apparently maximising the number of fledglings that they can raise. It is suggested that when there is a food shortage in the breeding season some passerines will lose more newly fledged chicks than normal whereas White-rumped Swiftlets in Fiji will lose more nestlings than normal. Nest size is not restricting clutch size as swiflets at Chillagoe did not raise more young when their nests were enlarged, and predators cannot be restricting clutch size because their nests are in total darkness. The swiftlets at Chillagoe are on the "mainland" yet produce a smaller clutch than those on the Fiji Islands. This is the reverse of predictions from the theory of "competitive release" on islands, therefore this theory cannot be used to explain the smaller clutch size of the birds at Chillagoe. The remaining factor is the food supply which is controlled by the occurrence of rain.The abundance of aerial insects was greater during days when rain fell. Adult swiftlets gathered less food in the dry season and in the dry periods between rain, and chicks put on more weight during rain periods, indicating that food was the critical factor restricting chick growth. Additionally, enlarged broods grew more slowly and never fledged more chicks than single-chick broods. This demonstrates that the abundance of food during the breeding season is the factor that not only regulates chick growth but also restricts clutch size. The food supply at Chillagoe does not last long enough for swiftlets to raise two single-chick broods, but it does last long enough for a unique strategy to have been developed which allows them to raise two chicks without producing a two-chick brood. This strategy involves the female laying the second egg after the first chick is fully feathered so that the first chick completes most of the incubation of the egg. The second egg hatches after the first chick fledges. The timing of laying the second egg leaves both parents free to forage for one chick only and allows them to raise two chicks in the shortened breeding (rainy) season that is characteristic of the savannah.
Content removed due to copyright restriction: Tarburton, MK (1986) . Breeding of the White-rumped Swiftlet in Fiji. Emu 86 , 214–227.
White-rumped swiftlet, Aerodramus spodiopygius, Chillagoe, Queensland, Fiji, Swifts breeding, Clutch size (bird eggs)