The effects of food size, pilfering and presence of a human observer on the caching behaviour of the South Island robin (Petroica australis australis)

摘要

Caching is the behaviour in which an animal stores food for later consumption. The most likely functions of caching are that it ensures food availability when conditions are severe, and it allows storage of energy in a form other than fat. The South Island robin (Petroica australis australis) is an endemic songbird that caches food items (such as insects) for later consumption. In this study, I examined caching in robins to address three questions. Firstly, I determined whether caching is related to prey size. I provided robins with mealworm (Tenebrio molitor) larva and recorded whether their propensity to cache was determined by prey size. As caching takes energy, robins should be selected to cache only the largest mealworms in which the energy return exceeds the costs of caching. I found that robins were significantly more likely to cache large mealworms while small mealworms were eaten immediately. However, there was no significant difference in the distance, height, or time taken to cache for caches of different-sized mealworms. Secondly, I determined how robins responded when their cached mealworms were pilfered by a human observer. After birds made 10 caches, I stole the three nearest caches while in sight of the caching bird. If birds perceived me as a pilferer, this should lead to future caches being stored higher up and further away from the observer. As expected, I found that robins stored food further away and more often out of sight when caching the next day in my presence. Lastly, I determined whether robins altered their caching behaviour in relation to the attentional gaze of an observer. To reduce the risk of pilfering, robins should alter their caching behaviour if being observed directly. I tested this hypothesis by altering my direction of gaze while birds cached. However, no differences were found in time taken to cache, caching distance or caching height in relation to my attentional gaze. Overall, my results indicate that caching food is dependent on food size and previous experience of cache pilferage but that more subtle cues, such as direction of gaze, are not used when robins decide on cache location. This work highlights the key roles of both prey size and risk of pilferage in the decision making process of whether or not a robin decided to cache a particular prey item, and if a cache is made, the location of these caches.