Estimating and Correcting for Bias in Population Assessments of Sooty Shearwaters

Abstract

We investigated the precision and accuracy of an infrared burrowscope for detecting sooty shearwater (Pufffinus griseus) chicks at 13 plots from 3 islands in southern New Zealand in 2003. We partially excavated burrows systems to reveal the entire burrow contents after 2 teams of observers had prospected all burrow entrances. Accuracy was similar between islands and observer teams at approximately 85%. The majority of the inaccuracy stemmed from failure to detect some chicks. Logistic regression modeling identified 4 burrow characteristics occurring between the entrance and the nest‐site that influenced detection of burrow occupants. Detection was lower at nest‐sites further from burrow entrances, in burrows with a high rate of burrow division, and in burrows with a high level of curvature. There was a positive relationship between the interaction of rate of division and curvature and detection of chicks. Distance from the burrow entrance was the only parameter that could be reliably used as a predictor of detection rate, so a reduced model containing only this variable was constructed to correct for burrowscope bias. The correction factor performed well on The Snares and Bench Island where predicted bias was very similar to observed levels (within 5%), but bias was overestimated on Putauhinu by up to 19.1%. Consistent bias, lack of damage to burrows from excavation, and the successful application of a correction factor all indicate the value of further testing burrowscope accuracy on other burrow‐nesting seabird species.

Predictive habitat modelling to estimate petrel breeding colony sizes: Sooty shearwaters (Puffinus griseus) and mottled petrels (Pterodroma inexpectata) on Whenua Hou Island

“…we are therefore confident that our use of entrance density counts to estimate population size is reliable. our use of aburrowscope (Hamilton 1998) and measured correction factors to account for failure to detect some eggs and chicks in deep burrows (Mckechnie et al 2007) further improved our estimates of population density. on the other hand, our use of a population model to estimate the total number of breeding pairs from the density of chicks may have introduced some errors by underestimating the proportion of adults that skip breeding each year (newman et al 2008).…”

“…occupancy measures for sooty shearwaters were studied on whenua Hou from 1997 to 2005 as part of the Kia Mau Te TītīMo Ake Tōnu Atu research programme . occupancy was corrected for the known under-detection rate in longer burrows (Mckechnie et al 2007). we then scaled this estimate of numbers of breeding pairs against a stable age distribution obtained from a sooty shearwater population model previously constructed by newman et al (2008).…”

“…36 cuthbert 2004. extensive surveys of burrow-nesting species are further hampered by the lack of reliable tools for detecting and identifying burrow occupants (ramos et al 1997;Hamilton 1998;Mckechnie et al 2007;rayner et al 2007).…”

Breeding parameters of the Sooty Shearwater (Ardenna grisea) on Long Island, New Zealand

“…The only similarly low density of Sooty Shearwater burrows was 0.09 burrows m -2 reported on Whenua Hou off Stewart Island (Charleton et al 2009). These densities are much lower than those reported on islands nearer the centre of the breeding range of the species, which vary from 0.98 burrows m -2 (McKechnie et al 2007) to 0.33 m -2 (Charleton et al 2009). The lack of burrow complexity on Long Island may be a result of the small population and low density of Sooty Shearwaters in this colony (Geary 2010;A.…”

A population and harvest intensity estimate for Sooty Shearwater, Puflinus griseus, on Taukihepa (Big South Cape), New Zealand

“…Determining burrow occupancy using a burrowscope is known to underestimate the true number of chicks present according to a decreasing function between probability of detection of the burrow occupant and its distance from the burrow entrance. We therefore adjusted the observed occupancy measures on each manu according to the detection correction function provided by McKechnie et al (2007).…”