Nest site selection of South Georgia Diving-petrels Pelecanoides georgicus on Codfish Island, New Zealand: implications for conservation management

Abstract

SummarySmall seabird species are often threatened by predation from invasive species at their breeding colonies and considerable efforts are invested into mitigating this threat. However, invasive predators may not be the only onshore threat affecting small seabird species. The South Georgia Diving-petrel Pelecanoides georgicus (SGDP) is a small seabird species, considered ‘Nationally Critical’ in New Zealand. Our objective was to identify terrestrial threats to the species at their sole remaining breeding colony in New Zealand, Codfish Island (Whenua Hou), following the successful eradication of invasive predators. To achieve our objective, we assessed the influence of five physical, three competition/attraction and three plant variables on SGDP nest site selection with generalised linear models (GLMs) and compared models using an information theoretic approach. Models including the distance to sea, slope, aspect, and sand flux outperformed other models and showed selection for steep seaward-facing foredunes with mobile soils. No invasive plant and competition/attraction variables were included in the best performing models. These results suggest that, due to the proximity of their preferred nesting habitat to the springtide line and the overall fragility of the foredunes, SGDPs on Codfish Island are extremely vulnerable to stochastic events and catastrophes, such as storms and storm surges. Eradication efforts directed at invasive predators on Codfish Island appear thus insufficient to safeguard this SGDP colony, necessitating further conservation strategies to secure the continued survival of this population.

Population growth estimates of a threatened seabird indicate necessity for additional management following invasive predator eradications

“…Various factors could be limiting WHDP population recovery. The distinct preference of the WHDP to only breed in fragile foredunes suggests that the species is extremely vulnerable to storms and storm surges (Fischer et al ., ). For example, a storm in October 2003 eroded the foredune in which the WHDP breeds, caused nest failures, and increased adult mortality (Cole, ).…”

“…All burrow counts were conducted by walking the entirety of the extremely small (approx. 0.018 km 2 ) colony back and forth (Taylor & Cole, ; Fischer et al ., ). After the first count in 1978 (the year in which the colony was discovered; Imber & Nilsson, ), all detected burrows were marked with stakes during each count (Cox, , R Nilsson pers.…”

“…We only considered counts of occupied WHDP burrows, because not all open burrows in the study area are occupied. We assessed burrow occupancy through stick palisades, sometimes in combination with playback/human mimics (Imber & Nilsson, ; Taylor, ; Taylor & Cole, ; Fischer et al ., ). As occupancy assessments require > 1 day, we excluded counts conducted within a single day.…”

“…) located circa 3 km west of Stewart Island (Rakiura), New Zealand. We conducted our study within the only WHDP colony, which is located in the Sealers Bay dunes (‐46.766˚ S, 167.645˚ E; Fischer et al ., , ). WHDPs breed in burrows in these dunes from early September to late January, but colony attendance varies with breeding stages (i.e., the species’ phenology), which complicates burrow counts.…”

“…To obtain a meaningful numerical value for p a,t , we monitored WHDP burrows during the entire 2017 and 2018 season ( n = 86 and 81, respectively). Specifically, we monitored the daily activity of these burrows using stick palisades (Imber & Nilsson, ; Taylor & Cole, ; Fischer et al ., ) to create an attendance curve throughout the season. At the same time, we monitored a subset of burrows in 2017 and 2018 ( n = 29 and 25, respectively) with a burrowscope (Taupe model, Sextant Technologies, Wellington, New Zealand; Lavers, Hutton & Bond, ) to assess mean dates of key phenology events (i.e., lay, hatch, and fledge dates).…”