Demographic and Social Influences on the Winter Ecology of a Migratory Songbird, the Golden-Crowned Sparrow (Zonotrichia Atricapilla)
Social groups can range from cohesive and stable to fluid and temporary, with many variations of the two. Social structure can effect selection pressures on individuals, and in turn, feedback from how social groups are organized can affect selection on different traits and behaviors. Here, I focus on group organization and how individual traits may correlate with sociality, and ultimately, survivorship. The research here ties together multiple threads to understand how individual traits connect to each other to shed light on how stable groups are organized, and which traits are most important to help maintain this structure over time. In Chapter 1, I ask if golden-crowned sparrows have consistent behavior within a season and over multiple years, and if this consistent behavior correlates with behavior in the field and several morphological traits. I found that the sparrows do have consistent behaviors in short-term captivity, yet these are independent of the traits measured here, and may connect with other factors such as predation response or foraging methods. For Chapter 2, I quantified individual sociality using social network measures and asked if dominance, plumage badges of status, sex, age, or mass could predict a sparrows’ social measures. I found that older and smaller birds had more connections. Chapter 3 links the previous chapters as I determined which behaviors and traits affected survivorship over ten years. Higher dominance increased survivorship, while older birds had lower survivorship. However, survivorship only decreased in the oldest age classes, showing evidence of senescence. Chapter 4 builds on Chapter 2, and used GPS tracking to discover if golden-crowned sparrows which were in the same winter communities bred in the same area. I discovered that sparrows in the same social community went to separate breeding locations, showing long-term social memory across migration. This thesis demonstrates that age, size and dominance are important aspects in social behavior and survival. Older individuals had more connections, and in conjunction with previous research, had stronger connections over years. This shows that older individuals can play a keystone role in maintaining cohesiveness of social communities over time.