Seabirds as environmental indicators: foraging behaviour and ecophysiology of common diving petrels (Pelecanoides urinatrix) reflect local-scale differences in prey availability

Seabird foraging behaviour can reflect prey abundance at sea, and is influenced by stress hormone levels, thus providing a potential indicator of at-sea conditions. Using common diving petrels (Pelecanoides urinatrix, hereafter CDPs), a procellariform that preferentially forages on crustacean zooplankton, we sought to understand how spatially separate colonies responded behaviourally and physiologically to contrasting prey levels with a view to recruiting this species as an environmental indicator. In 2016, incubating CDPs from Tiritiri Matangi (- 36.59S; 174.88E, low levels of preferred prey) and Burgess (- 35.91S; 174.12E, high levels of preferred prey) Islands within the Hauraki Gulf, New Zealand, were tracked using GPS devices. We hypothesised that Tiritiri birds would exhibit greater foraging effort and higher stress hormone levels across the breeding season due to lower levels of available prey. Hidden Markov methods were used to model foraging effort, and prey trophic level (stable isotopes: delta C-13 and delta N-15) and stress hormone levels (CORT) quantified in plasma samples. During incubation birds were spatially segregated when foraging. Tiritiri birds exerted more effort, chasing higher trophic level prey at larger distances from the colony, and had higher body weight and lower CORT than Burgess birds. However, bird CORT levels responded more to reproductive duties (peaking during chick rearing) as opposed to colony location, i.e. CORT was not consistently higher in Tiritiri birds. Although a snapshot, our findings illustrate the promise of integrating multiple parameters when recruiting seabirds as ocean indicators, resulting in improved resolution of future monitoring programmes based upon them.