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
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
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
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
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, artifici.ally 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.