Improving the accuracy of population size estimates for burrow‐nesting seabirds

Abstract

Seabird numbers can change rapidly as a result of environmental processes, both natural and anthropogenic. Informed management and conservation of seabirds requires accurate and precise monitoring of population size. However, for burrow‐nesting species this is rarely achieved due to spatial and temporal heterogeneity in burrow occupancy. Here, we describe a novel method for deriving more accurate population size estimates that employs mark‐recapture methods to correct for unknown variation in nest occupancy throughout a breeding season. We apply it to estimate breeding numbers of a colonial, burrowing seabird, the Little Penguin Eudyptula minor, on the Summerland Peninsula, Phillip Island, Australia. Estimates of active burrow numbers during the September 2008 to February 2009 breeding season were adjusted to numbers of breeding birds based on burrow occupation and modelled population demographics at six, fortnightly monitored reference sites. The population was estimated to be 26 100 (95% CI: 21 100–31 100) and 28 400 (23 800–33 000) breeding Penguins in two temporally separated surveys within one breeding season. We demonstrate using simulation that the method is robust to variation in burrow occupancy throughout the breeding season, providing consistent and more accurate estimates of population size. The advantage of using the corrected method is that confidence intervals will include the true population size. Confidence limits widened as burrow occupancy declined, reflecting the increased uncertainty as larger adjustments for low burrow occupancy were required. In contrast, the uncorrected method that uses burrow occupancy alone as a measure of breeding numbers was inconsistent and significantly underestimated population size across much of the breeding season. Although requiring considerably more survey effort, the corrected approach provides a more accurate means for monitoring population changes in colonially breeding animals while collecting demographic data that can help diagnose the drivers of population change.

Climate as a driver of phenological change in southern seabirds

“…Little Penguins are estimated to nest at this site (Sutherland and Dann 2012). At this site breeding and moult status were recorded.…”

The Little Penguin (Eudyptula minor) as an indicator of coastal trace metal pollution

“…St Kilda Little Penguins remain in Port Phillip Bay throughout the year and feed predominantly on Australian anchovy (Engraulis australis) (Preston et al, 2008), but their diet varies depending on prey availability (Kowalczyk et al, 2013). Phillip Island (38 30 0 S, 145 10 0 E) is located 140 km southeast of Melbourne, Australia, and has approximately 32,000 penguins nesting on the Summerland Peninsula (Sutherland and Dann, 2012). Phillip Island Little Penguins feed on a range of clupeoid fish (Australian anchovy, Pilchard Sardinops sagax, Barracouta Thyrsites atun, Red Cod Pseudophysis bachus) and Arrow squid (Nototodarus gouldi) in Bass Strait (Chiaradia et al, 2010), but during the winter months often forage within Port Phillip Bay, partially overlapping with the foraging area of penguins from the St Kilda colony (Chiaradia et al, 2012).…”

Estimating population size of a nocturnal burrow-nesting seabird using acoustic monitoring and habitat mapping

“…In addition, several seabird species nest in burrows or cavities, and are active in breeding colonies only during the hours of darkness. For these nocturnal burrow-nesting seabirds visual assessments of population size are not possible, and robust estimates of population size are usually based on marking a large number of birds (Sanz-Aguilar et al 2010;Sutherland and Dann 2012) or estimates of burrow occupancy (Pearson et al 2013). On many islands the marking of seabirds or inspection of burrows is logistically not feasible, because birds nest on inaccessible cliffs.…”