Release of constraints on nest-site selection in burrow-nesting petrels following invasive rat eradication

Abstract

Introduced mammals have been eradicated from many offshore islands around the world, removing predation pressure from burrow-nesting seabirds and other affected wildlife. Nest-site selection in procellariiform seabirds is mediated by nesting habitat characteristics and social information, although it is unclear if, or how, nest-site selection will affect post-eradication colony growth. Using a Bayesian hierarchical modeling approach we assessed how nest-site selection differs among burrow-nesting seabird colonies at different stages of recovery after Pacific rat (Rattus exulans) eradication. We compared nest-site selection in a community of seven procellariiform species among six offshore islands in northeastern New Zealand: four designated rat-free over a continuum within the last 26 years, an island which never had rats, and an island with rats present. We hypothesized that, immediately after eradication, birds would be constrained to nesting habitat where they were less vulnerable to predation, and as time since eradication increased birds would eventually spread to new habitat. We found a positive relationship between mean burrow density and time since rat eradication. Soil depth was the most important predictor of burrow presence, abundance, and occupancy in plots among islands, with more burrows found in deeper soil. We found that the relationships between habitat covariates and nest-site selection decreased with increasing time since eradication. The probability of a covariate having a significant effect on nest-site selection decreased with increasing time since eradication and decreasing variability in the covariate across an island. Our results suggest that the eradication of rodents reduced constraints on petrel nesting distribution and that nest-site selection in burrow-nesting petrels may be influenced by burrow density, where selection of particular nesting habitat characteristics may be relatively more important in small recovering populations. We conclude that colony expansion immediately after predator removal is complex, influenced by numerous interacting factors, but may be partly limited by the availability of suitable nesting habitat.

Spatio-temporal changes in density and distribution of burrow-nesting seabird colonies after rat eradication

“…We infer that, although nesting habitat is important in determining the distribution and density of petrel burrows, time since rat eradication still contributed to the final pattern of observed burrow distribution. We therefore conclude that although there may be interactive effects between colony recovery and nesting habitat quality, and that habitat should not be ignored, the effects of habitat are not necessarily enough to restrict increases in colony growth after eradication (Major et al 2011;Buxton et al 2015).…”

“…(2) where β tse was the rate at which burrow density increased with the number of years after eradication, and β lk was the effect of nesting habitat variation among islands k and environmental covariates l. Environmental covariates included soil depth, southern and western aspect, slope, rock cover, karamū (Coprosma macrocarpa) stem count, māhoe (Melicytus ramiflorus) stem count, and total stem count, and were selected based on habitat selection models (Buxton et al 2015). The intercept (α jk came from a prior distribution of hyper-parameters Μ α and σ…”

“…Environmental covariates are known to affect the density and distribution of burrow-nesting seabirds (Buxton et al 2015). Abiotic and biotic factors determining nesting habitat quality often include soil type, which must be soft enough to excavate, but strong enough to avoid collapse; canopy and understorey vegetation, which adults must penetrate safely to reach their burrow; ground cover, which must be avoided or burrowed under; slope, which affects soil drainage; and aspect, elevation, and topography, which affect the ease of take-off and landing (Burger & Gochfeld 1991;Thompson & Furness 1991;Rodway et al 1998;Bancroft et al 2005a).…”

An estimate of the South Georgia diving petrelPelecanoides georgicuspopulation at Ile de la Possession, Crozet archipelago

“…On Ile aux Cochons, feral cats ( Felis catus ) were also introduced, and are still present, and they may predate on diving petrels. The removal of introduced predators including the Pacific rat ( Rattus exulans ) on islands off the north-eastern coast of New Zealand's North Island resulted in an increased population of burrow-nesting seabirds, including common diving petrels (Buxton et al 2015). At Marion Island, South Georgia, common diving petrels are scarce, probably affected by the introduction of cats in the 1950s (Dilley et al 2017).…”

A GIS‐based decision‐making approach for prioritizing seabird management following predator eradication

“…The resulting weights used for the vegetation attribute were forest 45%, shrubland 41%, and grasslands 14%. The remaining attributes relating to soil qualities among associated burrowing seabirds were weighted toward friable, low induration soils on 5°–26° slopes (Table 1; Bancroft et al ; Whitehead et al ; Buxton et al ; Table S1 of Appendix S1). For example, gray‐faced petrels ( Pterodroma gouldi ) are known to burrow in forests and grasslands, in sandy, low non‐indurated soils on slopes between 10° and 40° (Whitehead et al ).…”