Zebra finch behaviour differs consistently between individuals but is not affected by early life adversity

Individual variation in animal personality is ubiquitous, but little is known of its proximate causes. Theory predicts individuals will be more risk prone when their life expectancy is short, and adverse environmental conditions during development shorten life span. We modified developmental conditions of zebra finches, Taeniopygia castanotis, by manipulating parental foraging conditions to be either ‘harsh’ or ‘benign’ and studied offspring behaviour in adulthood using multiple tests: exploration, novel object, tonic immobility, dominance and sociality. Each test was done in duplicate, and repeatabilities of test scores ranged from 0.31 to 0.55 independent of treatment with one exception: tonic immobility was repeatable in individuals reared in harsh conditions only. Correlations between behaviours (i.e. behavioural syndromes) were generally weak and nonsignificant. Growing up in harsh conditions and in larger broods both negatively affected offspring growth and thereby presumably their life expectancy, but neither affected behaviour in standardized tests. This raised the question whether the origin of variation in personality was perhaps largely genetic, but heritability estimates were low to moderate. We conclude that variation in personality in our study can be attributed to environmental effects, but independent of early life adversity as manipulated here. To explain this finding, which runs counter to our expectation, we present two hypotheses. First, the adolescent social environment impacted the ontogeny of personality, overshadowing developmental effects. Second, the risk associated with a behaviour may be state dependent, with identical behaviours encompassing a greater risk in low-quality than in high-quality individuals. Thus, while individuals with different developmental backgrounds displayed similar behaviour, the perceived risk this entailed may have been very different and in accordance with theoretical predictions. Given that we found no behavioural syndromes, a theory that assumes behaviours to be correlated does not apply, and we discuss alternative functional explanations for individual differences in behaviour.