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Foraging behaviour can have a major influence on the survival and reproduction of individuals which can ultimately impact the viability of a population. Foraging is particularly challenging for procellariiformes (tube nosed seabirds) who feed on patchily distributed prey in the highly dynamic marine environment. During the breeding season procellariiformes must also increase their foraging effort to raise their chick whilst having a reduced foraging range. As a result, procellariiformes have adopted various foraging strategies, such as dual foraging and sexual foraging dimorphism, to cope with this energy demanding lifestyle. Westland petrels (Procellaria westlandica) are an endangered winter breeding procellariform endemic to the West Coast of New Zealand’s South Island. Unlike other procellariiformes, previous studies have found little evidence of Westland petrels using sexually dimorphic or dual foraging strategies. Furthermore, Westland petrels also display a high level of individual variation in foraging behaviour. To understand why there is so much variation and what factors are driving it, I first examined variation at the population, individual and within individual level to describe and categorise different foraging strategies. I then investigated how factors such as year, sex and foraging site influenced variation. Finally, I examined how oceanic variables influenced habitat selection and foraging characteristics to understand how the environment drives variation in foraging behaviour.   Considerable variation was found at all levels. Most of the variation was explained by year with individuals taking shorter foraging trips in 2011 and longer trips in 2015. Females foraged further than males suggesting that there is some degree of sexual foraging segregation occurring in Westland petrels. I also found that the highest variation in foraging behaviour was exhibited by individuals within their core foraging site on the West Coast. Sea surface temperatures were highest at the West Coast foraging site and individuals within this site showed differences in habitat selection among years. Habitat selection at the West Coast site also differed between sexes suggesting that males are outcompeting females for prime foraging spots.  Overall, my results indicate that foraging conditions on the West Coast are highly variable likely due to rising sea surface temperatures, marine heatwaves, and the effects of the El Nino-Southern Oscillation. As a result, it is likely that prey availability on the West Coast is unpredictable causing high variation in foraging behaviour and sexual foraging segregation. With climate change, foraging conditions on the West Coast are predicted to get more unpredictable as sea surface temperatures continue to rise and extreme weather events become more frequent. These factors will make foraging increasingly difficult for Westland petrels and could see them rely more on fishery discards as a source of food, increasing their risk of incidental mortality. Conservation management should focus on protecting the petrels core foraging area around the Hokitika canyon to help limit the effects of climate change. Fishery management should also focus on limiting or prohibiting offal discards to prevent the incidental mortality of Westland petrels.

Westland petrel (Procellaria westlandica) foraging behaviour: Patterns and drivers of individual variation