D’Arros shearwater colony grew nearly 10-fold after rats were eradicated

D’Arros Island in Seychelles has recorded a sharp rise in its Wedge-tailed Shearwater breeding population after rats were eradicated in 2003. Researchers counted 2,768 breeding pairs in 2016 and 2,406 in 2021, compared with about 254 pairs recorded in 2009.

The colony also spread across a larger area, while breeding burrow density increased during the later surveys. According to research published in Marine Ornithology titled ‘Rapid increase in size of Wedge-tailed Shearwater Ardenna pacifica colony following rat eradication’, the results indicate a true population increase. The researchers noted that differences in census timing and methods affected comparisons, but the larger colony area and higher burrow density supported the finding.

How did rat eradication affect Wedge-tailed Shearwaters on D’Arros Island

The researchers estimated 2,768 breeding pairs on D’Arros in November 2016 and 2,406 pairs in November 2021.

Both figures were nearly ten times the approximately 254 breeding pairs estimated in 2009. The 2016 estimate had a 95% confidence interval of 2,424-3,112 pairs, while the 2021 estimate had a 95% confidence interval of 2,143–2,667 pairs.The study described the change as a rapid increase in colony size following rat eradication. D’Arros had been rat-free since 2003, giving the researchers a 13-year gap between eradication and the first new census.

They also found that the colony occupied a larger area and had higher nesting density, supporting the conclusion that the population increase was genuine.

What changes were recorded in the Wedge-tailed Shearwater colony

The total Wedge-tailed Shearwater colony covered 3.40 hectares across six colony sites in 2016. By 2021, it covered 3.85 hectares across seven sites. The later estimate included a very small nesting area in the northwest of the island that had not previously been identified.

The researchers also found Wedge-tailed Shearwaters vocalising at night inside dense coconut palm vegetation during both surveys, but no nests were found there, so those areas were excluded from the census.Breeding burrow densities ranged from 21 to 1,866 pairs per hectare in 2016 and from 17 to 2,391 pairs per hectare in 2021. The increase in total colony area and burrow density helped confirm that the population had increased.

How did researchers estimate the number of breeding pairs

The study combined direct counts with area-based estimates. Researchers used direct counts in areas with relatively low burrow densities and area-based estimates in high-density areas where walking could collapse closely spaced burrows. In those areas, they selected 50-square-metre plots and inspected burrows for signs of occupation.A burrow-scope helped researchers inspect deep and convoluted burrows, with some extending more than 70 centimetres below the ground.

In 2016, researchers also used call playback to confirm the status of burrows that appeared empty but showed signs of recent activity. The researchers noted that differences in sampling methods made direct comparisons with the 2009 census difficult.

Researchers identify November as the peak incubation period on D’Arros

The researchers examined burrow status from October to December 2021 to identify the peak incubation period on D’Arros. They found that November had a significantly higher proportion of burrows occupied by incubating birds than October or December.

In the high-density plots, the average number of total breeding burrows was seven in November, compared with two in October and one in December.In the low-density area, seven breeding burrows were recorded in November, compared with six in October and seven in December. The researchers said identifying the peak incubation period was essential for future comparisons of population estimates.