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Breeding strategies and community structure in an assemblage of tropical seabirds on the Lowendal Islands, Western Australia

Nicholson, Lisa (2002) Breeding strategies and community structure in an assemblage of tropical seabirds on the Lowendal Islands, Western Australia. PhD thesis, Murdoch University.

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Information about tropical seabird communities is less abundant in the literature than for those that occur at higher latitudes. In research papers examining seabird breeding communities in tropical environments, it has been found that food availability was temporally and spatially variable in comparison to higher latitudes. This environmental variability would be expected to influence the life-history traits of tropical seabird species, and in turn, the structure of the communities in which they occur. To examine the impact of environmental variability upon the life-history traits and community structure of tropical seabirds, a comparative study of the breeding strategies of three tropical tern species and an outlier, shearwater species was carried out, at the Lowendal Islands, Western Australia (20 degrees 39'52S; 115 degrees 34'44E) between 1996 and 2000.

Bridled Terns Sterna anaethetus and Wedge-tailed Shearwaters Puffinus pacificus shared the life-history traits of migration or dispersal from the breeding colony and east-Indian Ocean metapopulation when not breeding, a summer breeding schedule, nest-site fidelity, concealed nest-sites, clutch size of one, offshore and pelagic foraging regime, and protracted post-natal growth. Crested Terns Sterna bergii and Lesser Crested Terns Stern bengalenis bred within the same colony and shared the life-history traits of presence at the breeding colony when not breeding, an autumn breeding schedule, change in colony site each season, open dense nest-sites, brood size of one. inshore foraging regime (as well as offshore for Crested Terns) and rapid post-natal growth.

It was found that each species experienced variation in nest-site occupancy and/or colony size, as well as variation in timing of breeding and breeding success between years. The effects upon each species seemed to operate independently, as concurrently breeding species did not appear affected by the same events, with the exception of cyclones. Late and/or poor breeding success in a species often coincided with oceanographic changes in the north-west region, such as fluctuations in the strength of flow and temperature of the Leewin Current. The Leewin Current is a warm water, low salinity current flowing southwards along most of the Western Australian coastline and has a stronger, warmer flow during El Nino Southern Oscillation (ENSO) events. Cyclone activity is also influenced by ENSO induced changes in the region's water temperature. Changes in the Leewin Current affected the timing of breeding for Wedge-tailed Shearwaters, and feeding conditions for all study species, presumably as a result of changes in prey availability, while cyclone events delayed breeding for Crested Terns and Lesser Shearwaters in the years studied. Cyclone events were the most obvious cause of re-laying in the case of Crested Terns and Lesser Crested Terns, and breeding failure in the case of Bridled Terns and Wedge-tailed Shearwaters. Breeding failure was common among later breeders in the Crested Tern colonies. Breeding success was relatively high for lesser Crested Terns in all years studied, as they hatched and fledged their chicks within the Crested Tern colony among the first wave of breeders.

There was some overlap in the diets and foraging grounds of the four study species. Crested terns and Lesser Crested Terns had the most similar diet, however, Crested Terns appeared to be the most opportunistic foragers, with the highest diversity of dietary prey, while Lesser Crested Terns diet consisted of a high proportion of atherinids. Bridled Terns has the next highest prey diversity, while half the diet of Wedge-tailed Shearwaters in this region, similar to conspecifics elsewhere, consisted of squid. Wedge-tailed Shearwaters were observed to be offshore and pelagic foragers. Their distribution in the waters surrounding the Lowendal Islands ranged to the full extent of observation transects(63 Km), and most likely beyond, excluding inshore waters (i.e. within 5 km of islands). They were not present in the vicinity of the Lowendal Islands when they were not breeding. Bridled Terns were observed to be offshore foragers, also ranging to the full extent of transects, and most likely beyond, occasionally including inshore waters. They were rarely observed when not breeding, with the exception of a small number of fledglings at the end of the breeding season. Crested Terns were observed to combine inshore and offshore foraging grounds, only ranging to the full extent of transects during their breeding season. Lesser Crested Terns were observed foraging inshore only.

Chick provisioning, in terms of meal size and frequency, varied between years and within seasons for all species. It was demonstrated that there were differences in mean corrected meal sizes between years for Wedge-tailed Shearwaters, and the same was inferred from chick growth curves for Bridled Terns. Wedge-tailed Shearwaters provisioned their chicks with larger meal sizes and a higher occurrence of double feeds in 1996/97 than in other years studied. The amount of time spent feeding chicks in the burrow by Wedge-tailed Shearwaters adults became shorter as the breeding season progressed. It was demonstrated that all three tern species increased the meal size delivered to chicks as they grew. Wedge-tailed Shearwaters did not increase meal size as the season progressed, however they did increase feeding frequency. The feeding frequency of Crested Terns was highest in 1999, nearly doubling that observed in 1997 and 1998. In 1998 Lesser Crested Terns fed their chicks up to six times more frequently than Crested Terns.

Chick growth varied for Wedge-tailed Shearwaters and Bridled Terns between seasons. Bridled Tern chicks grew more rapidly in 1997/98, were heavier and reached higher asymptotes for all linear parameters, the exception of wing length, than in 2000. On the other hand, Wedge-tailed Shearwater chicks grew more rapidly in 1999/2000 than in 1997/98, however, they attained similar asymototes for all parameters in both years. In the absence of repeat measures of same individuals, Crested Tern and Lesser Crested tern growth was examined using age categorization based on wing and tail feather development. Both species appeared to attain similar sizes at similar rates in each year.

Wedge-tailed Shearwaters accumulated weight up to 30% in excess of mean adult weight in all seasons. This is a common trait exhibited in procellarigorms, and some weight loss occurs prior to fledging. It is suggested, based upon evidence from other studies in which weight loss prior to fledging was found to be water loss, that lipid accumulation acts as an insurance for the period after fledging when young are leaning to forage for themselves.

The information gathered for this project found no direct evidence of competition among the species comprising the seabird community if Lowendal Islands. Niche overlap occurred amongst all species, particularly in the case of lesser Crested Terns and Crested Terns, which shared breeding phenology, breeding colonies, foraging grounds and dietary overlap. Niche overlap also occurred between Wedge-tailed Shearwaters and Bridled Terns, which shared breeding phenology and foraging grounds.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Supervisor(s): Bradley, Stuart
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