Preparing for translocation: chick development, provisioning regimes and an experimental translocation of mottled petrels, Pterodroma inexpectata

Abstract

Mottled petrels, Pterodroma inexpectata, are mid-sized burrowing seabirds endemic to New Zealand that play a key role in driving terrestrial biodiversity by providing marine-terrestrial nutrient linkages. Mottled petrels once bred in high numbers throughout New Zealand, though are now considered ‘near threatened’ due to recent human-driven habitat loss and predation, and have a limited distribution on a small number of islands south of New Zealand. Because of their former distribution, mottled petrels are a popular candidate for restoration projects aimed at enhancing species populations and restoring ecological links between land and sea. However, before these translocations can proceed, critical knowledge of mottled petrel breeding biology, and assessment of the appropriateness of current translocation techniques are required to guide species-specific restoration management protocols. This study aimed to characterise previously unknown aspects of breeding biology critical for the successful translocation of mottled petrels. The breeding biology of mottled petrels, focussing on chick development and provisioning regimes, was investigated during the entire chick-rearing period on the island of Whenua Hou (Codfish Island), New Zealand during 2013. Additionally, late-stage mottled petrel chick growth and provisioning regimes were investigated, and an experimental translocation of mottled petrel chicks to assess the efficacy of standard translocation practices was undertaken on Whenua Hou during 2012. These data allowed inter-annual comparisons of chick development and provisioning regimes, and comparison with other species of gadfly petrels (genus Pterodroma). The weight and morphometric parameters (wing chord length, minimum bill depth and tarsus length) of mottled petrel chicks investigated appeared to follow typical growth patterns for gadfly petrels. However, an inter-annual comparison of late-stage mottled petrel chicks revealed that mottled petrel chick growth in 2013 was retarded and chicks fledged in poorer condition (mean weight 290 ± 9 g) than in 2012 (mean weight 351 ± 6 g), despite provisioning rates and meal sizes remaining consistent between years. It is postulated that the differences in chick condition between seasons were primarily the result of differences in diet between seasons, with prey in 2013 dominated by crustaceans (krill) of lower calorific value than in 2012 when prey probably consisted of more fish and squid. It is likely that the availability and distribution of prey in 2013 were altered due to increases in sea-surface temperature associated with the El Niño Southern Oscillation in the Southern Ocean. Examination of provisioning regimes showed that mottled petrel chicks were fed less frequently (on average one feed every six days) than other gadfly petrels of a similar size (one feed every two to three days), presumably as a result of their high-latitude foraging niche which would require extended travel times. A major, though unexpected, finding of this study was that daily handling retarded chick growth (mean fledging weight = 235 ± 14 g and wing chord = 238 ± 6 mm) compared with chicks handled less frequently (mean fledging weight = 290 ± 9 g and wing chord = 253 ± 3 mm). In 2013, nutritional stress may have further exacerbated the effects of handling stress. Mottled petrel chicks that underwent an experimental translocation in 2012 fledged in good condition, with mean fledging weight equal to mean adult weight (325 g), which bodes well for future efforts aimed at restoring this species within its former range. This research fills some gaps in our knowledge of the breeding biology of mottled petrels: knowledge which is critical for the successful translocation of this species. It indicated that the optimal age to translocate mottled petrel chicks is 20 to 15 days before fledging. A wing chord length range of approximately 230 to 245 mm should identify chicks of this age, and it is recommended that the selected chicks are within the weight range of 460 - 520 g. Translocated chicks should be provisioned with approximately one 35 g meal of the standard sardine diet every three days until fledging, with a target fledging weight of approximately 350 g. It is expected that these findings will benefit the conservation management of mottled petrels, including the refinement of translocation practices specific to mottled petrels. Further, this research highlights the significant influence of inter-annual variation in environmental conditions on the breeding biology of seabirds such as mottled petrels. This information reinforces the importance of long-term monitoring programmes of top predators, including mottled petrels, which provide an integrated view of the consequences of oceanic anomalies throughout trophic levels. Such knowledge could allow us to forecast the potential effects of global climate change on seabird communities during the near future, and help guide pre-emptive conservation action in the face of population declines.