PhD defence Stephen Salazar

Stephen Salazar (BPE)

Promotores: Prof. J. Komdeur, Prof. P. Korsten (Aberystwyth University)

Individual risk and reward in the wild

A study on blue tits

Behavioural biologists seek to understand why individuals of the same species differ in their behaviour. The "pace-of-life syndrome" (POLS) hypothesis offers a framework to explain such individual behavioural variation. It puts forth that individuals differ broadly in how they take risks in their day-to-day behaviour as well as in their overall life-history strategies, suggesting that the two are linked. This thesis examines such theoretical predictions using a wild population of blue tits (Cyanistes caeruleus), focusing on two key naturally relevant contexts: territorial aggression in males and risk-taking by parents in the face of predation. The findings reveal differences among males in aggression and risk-taking but show no correlation between these behaviours across individuals. By manipulating predator threat levels (using woodpecker and sparrowhawk taxidermy mounts), the research further tested whether risk prone individuals are less flexible in their behaviour. Although the population demonstrated behavioural plasticity by a stronger response towards the high-level threat (sparrowhawk), individual differences in plasticity were not observed. Over three years, the study also tested whether aggressive males prioritize current reproductive success over future reproductive prospects. The results provided only weak evidence linking aggression to reproductive success or survival, suggesting that aggression may not be a major factor in these reproductive fitness trade-offs. Subtle year-specific relationships hinted at fluctuating natural selection on aggressiveness and components of reproductive success. Overall, the thesis offers limited support for a unified pace-of-life axis encompassing individual life-history and behavioural variation, while highlighting the importance of studying natural behaviours in wild populations to better understand the evolution of behavioural differences.

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