Energy management and foraging behaviour in food hoarding paridae

Abstract

Winter poses significant energetic challenges for small tit species at northern latitudes, where food scarcity and shorter daylight hours limit foraging opportunities, whilst energy demands increase with dropping temperatures. Several bird species in the Paridae family hoard food to improve winter survival. How this behaviour is mechanistically regulated—both seasonally and in terms of foraging decision-making—is not fully understood. This thesis investigates energy management in hoarding tit species across three projects. The first project focuses on the optimisation of energy management through fat reserves. According to Optimal Body Mass theory, hoarding birds are expected to carry less fat in winter than non- hoarders, as they have more predictable food resources. My findings show winter fattening patterns in both hoarding and non-hoarding tit species in the UK, with less pronounced patterns for hoarding birds. Furthermore, I explore the effects of temperature and day length on fattening, showing that day length is a better predictor of bird body mass than recent temperature. My second project explores the seasonal patterns of hoarding, focusing on how food availability affects hoarding intensity during winter. I hypothesise that hoarding motivation might remain high in winter, but that birds reduce hoarding behaviour as they do not have enough food available. To gather data, I developed a large citizen science initiative where volunteers across the UK, USA and Canada reported the frequencies of hoarding and eating behaviours among birds visiting their garden feeders. Results reveal similar but slightly delayed hoarding patterns in food-supplemented birds compared to those in the wild, suggesting food supplementation was not sufficient to prevent the decline in hoarding, or that hoarding motivation is lower in winter. My third project explores the variables influencing foraging decisions, specifically how birds decide whether to consume or hoard food. I develop agent-based models in which decision- making is based on stomach content, fat reserves or their combinations. I then explore how agents with different decision rules perform in varying environments and how this is reflected in their physiology and behaviour. The model outcomes demonstrate that simple decision rules, in which birds only hoard when there is a surplus of food and respond to information about their fat reserves, effectively replicate observed hoarding and retrieval patterns of short-term hoarding in winter.