Foraging is a central theme in ecology and underlies some of the most fundamental interactions among species. While diet has long been treated as uniform within species, a paradigm shift is now taking place as ecologists recognize that individuals vary. However, questions remain about why this intraspecific variation exists and what consequences it may have for individuals, populations, and ecosystems. This information may be particularly important for species that are threatened due to food limitation, as dietary variation could be used to inform conservation efforts.

In this thesis, I used the hihi (Notiomystis cincta), a threatened New Zealand passerine, as a case study to explore the causes and consequences of dietary variation and possible implications for conservation. Because observing threatened species is often difficult in the wild, I first developed methodology to obtain more reliable dietary estimates from limited foraging observations. Using these improved metrics, I found that individual hihi specialize within the broader species diet. Generalists survived better than specialists, suggesting that dietary specialization may have consequences for fitness.

I next explored whether an array of intrinsic and extrinsic factors could explain dietary variation. Results suggested a weak correlation between diet and sociality, so I then conducted an experiment to test how directly altering the social environment affects diet. Using two scheduled conservation translocations, I found that individual hihi shifted towards more generalist diets when they were moved from a highly competitive source site to a less competitive release site. This suggests hihi share the same inherent dietary preferences but must specialize to avoid competition in dense populations. Furthermore, individuals that were more generalist before the translocation were more likely to be seen five months later, but only if they remained at the source site; at translocated sites, generalists tended to be less likely to be seen than specialists. These results suggest that selectively translocating specialists may improve translocation success without jeopardizing the source population.

Finally, I investigated how foraging behavior can shape the environment. Hihi are pollinators of several threatened native plant species, so conservation of hihi could promote ecosystem recovery as well as species recovery. However, little is known about how effectively hihi translocations restore lost pollination function and whether individuals vary in their pollination abilities. Through a multi-site pollinator exclusion experiment, I found that ecosystems may be able to partially adapt to the loss of hihi. Nevertheless, hihi bring unique benefits to pollination by improving seed quality. There was also some evidence that individual hihi vary in their efficiency as pollinators.

Together, my findings demonstrate that foraging is inextricably linked to the environment: the ecosystem shapes individual variation, which, in turn, shapes ecological functions. A better understanding of dietary variation may also have important implications for threatened species and ecosystems, informing the design of conservation interventions.