Individual Variation In Stress Hormones And Behavioral Profiles Represented By Personality And Plasticity In Tree Swallows

Behavioral and evolutionary ecologists have been increasingly interested in why individuals from the same population exhibit consistent differences in behavioral and physiological traits across contexts - a phenomenon referred to as ‘animal personality.’ Both the repeatability of traits and the extent to which the traits vary with the environment (between-individual plasticity) should be considered aspects of the animals’ personality.However, only recently have researchers begun to discover the potential underlying factors that influence individual variation of behavioral and physiological traits. Because these behavioral traits have been shown to be heritable, related to fitness, and shaped by evolution, they may represent strategic behavioral types (i.e., stress-coping styles) that enable individuals to cope with their environment. Thus, how an individual’s physiological system responds and adapts to ecologically relevant environmental stressors can affect not only their behavioral response but more importantly their fitness. In the present study, I used a reaction norm approach to assess if baseline and stress-induced corticosterone (CORT) and nest defense behavior are components of personality and plasticity (‘E’, ‘I’, ‘I X E’) and how within- and between-individual variation of these components respond to ecologically-relevant environmental variation (weather, site), breeding stage (early versus late nestling rearing stages), and individual characteristics (e.g., age, body mass) in a wild population of adult female tree swallows (Tachycineta bicolor). Although baseline CORT was not repeatable, stress-induced CORT was moderately repeatability (r= 0.330; ‘I’) while nest defense behavior displayed high repeatability (r= 0.445; ‘I’). Similarly, baseline CORT was very plastic while stress-induced CORT and nest defense behavior exhibited low to moderate plasticity. Personality was associated with the extent to which individuals adjust their phenotype as function of changing conditions (behavioral plasticity); individuals with higher stress-induced CORT had more plastic stress-induced CORT and more aggressive individuals had more plastic behavior. Together these results suggesting that behavioral and hormonal personality can constrain plasticity in those traits. Although I found no direct links between CORT and nest defense behavior, my study highlights the importance of repeatedly measuring individuals while they experience ecologically-relevant stressors. Future studies should investigate how such within- and between-individual variation in personality and plasticity influences fitness