Pruett-Jones & O'Donnell: Land Birds on Barrow Island 
yet we recorded just one individual on our quadrats. This 
species, and two other aerial insectivores (Welcome 
Swallow and White-breasted Woodswallow) appear to 
have declined in numbers. Assuming this trend relates to 
insect abundance, it is not yet clear whether it is a short¬ 
term trend reflecting annual variation in insect 
populations or a long-term trend, with broad ecological 
implications. If, on Barrow Island, the diet of Australian 
Kestrels is primarily insectivorous, it could be said that a 
total of four insectivorous species have shown population 
declines. Notably, however, the three most abundant 
species (Spinifexbird, White-winged Fairy-wren, and 
Singing Honeyeater) are either obligatory or primarily 
insectivorous, and the populations of these species have 
not declined (Table 2). 
Despite the distance of Barrow Island from the 
mainland, and the relatively long time it has been 
isolated, there is only one endemic subspecies of bird 
that resides there, the White-winged Fairy-wren (M. /. 
edouardi, Ford 1987; Schodde and Masonl999) and genetic 
analysis (Driskell et al. 2002) has confirmed the genetic 
basis of this subspecific designation. Another unique 
subspecies of the White-winged Fairy-wren (M. /. 
leucopterus) is found on Western Australia's largest 
island, Dirk Hartog Island. Wooller et al. (1985) suggested 
that the Barrow Island population of the Singing 
Honeyeater may be an endemic subspecies, based on 
vocalizations and measurements, but Schodde and 
Mason (1999) did not separate the Barrow Island 
population from the mainland subspecies L. v. sonorous, 
although the basis for that decision is unclear. No genetic 
analysis of Singing Honeyeaters from Barrow Island has 
been conducted. 
Habitat relations 
In this study we did not examine specific habitat 
preferences of individual species, but rather broad 
differences in habitat types and how these related to 
species richness and abundance of birds. All habitat types 
on Barrow Island, with the exception of clay pans, 
drainages, and coastal areas are dominated by spinifex. 
All species that regularly feed on the ground or in 
vegetation, e.g, Spinifexbird, Singing Honeyeater, and 
White-winged Fairy-wren, used spinifex plants, but these 
species also seemed to prefer habitats in which spinifex 
was interspersed with shrubs and trees. Across habitat 
types, species richness was correlated with maximum 
vegetation height, indicating that more structurally 
diverse habitats supported more species. Furthermore, 
Spinifexbirds were more abundant in areas with greater 
cover and more emergent shrubs, and Singing 
Honeyeaters were more abundant in areas with taller 
vegetation. Spinifexbirds preferred the densest vegetation 
of any of the species, and rarely emerged from the 
spinifex. The greater density of Spinifexbirds in areas 
with emergent shrubs may be related to variation in 
insect abundance in areas of greater plant diversity. 
Effects of oil-field development 
The effect of the oil-field development on land birds 
on Barrow Island is difficult to accurately assess in a 
short-term study such as ours. Approximately 4% of the 
land area on Barrow Island has been altered, but this has 
had both positive and negative effects on the land birds. 
Positive effects include more nesting sites for some 
species {e.g., swallows and woodswallows nesting on 
buildings, ospreys nesting on utility poles) and increased 
availability of fresh water for some species {e.g., Zebra 
Finches), whereas negative effects include loss of habitat, 
altered habitat around man-made structures, etc. 
In our comparison of species diversity in natural areas 
and altered areas, habitat along road sides supported 
more individuals (but not more species) than natural 
areas. We believe the reasons for this difference had to 
do with the effects of disturbance in a Triodia dominated 
habitat. When an area was disturbed, it appeared that the 
largest species of Triodia on Barrow Island, T. angusta, 
rapidly colonized the disturbed area, often times 
regardless of the dominant surrounding vegetation. For 
example, in limestone ridge areas, where the small 
(approximately 0.3-0.5 m high) T. wiseana dominated 
natural vegetation, along the roads there was a two to 
three m wide area on either side of the road where T. 
angusta occurred, often up 1.5-2.0 m high. Larger, and 
denser T. angusta provided suitable habitat for more 
individual birds. 
As a Class A Nature Reserve, no camping or landing 
by boaters is allowed on Barrow Island. The only allowed 
human activity on the island is that associated with the 
oil-field development and the present leaseholder 
(Chevron Inc.) controls all human activity on the island. 
To date, that control and strict quarantine practices for 
people and vehicles traveling to the island, has acted to 
protect the Barrow Island ecosystem. Barring the obvious 
possibility of a damaging oil spill, the avifauna on 
Barrow Island and the ecosystem itself should continue 
in its relatively undisturbed condition. When the oil-field 
is decommissioned, which is likely within the next 10-20 
years, the decision on the future of Barrow Island will 
obviously set the stage for any possible future changes. 
Given the unique ecosystem on the island, great care 
should be taken to insure that no future human activity 
threatens the island. 
Acknowledgements: We wish to thank Chevron Inc., Troy Crawford, 
Stefan Fritz, Les McClements, and Luke Ulstrup, for logistical or other 
support on Barrow Island, Harry Butler for access to unpublished 
records, and Harry Butler, Ron Johnstone, Melinda Pruett-Jones, and two 
anonymous reviewers for comments on the manuscript. 
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