Intruder sex and breeding stage influence territorial aggressiveness: But only in male not female blue tits (Cyanistes caeruleus)

Aggressive behaviour is common in nature and may be closely linked to individual fitness (i.e., survival and reproductive success). Aggressive individuals may gain priority access to resources such as food and mates. At the same time, they may also bear costs, including energy expenditure, risk of injury,exposure to predation, and physiological stress. At present, research on individual differences in aggressiveness and its ecological causes and consequences remains scarce. Moreover, much research has focused predominantly on aggressiveness in males rather than females. In this study, we tested the repeatability of aggressiveness in both sexes in a nestbox-breeding blue tit (Cyanistes caeruleus) population using simulated territorial intrusions. We investigated female and male aggressiveness towards intruders of both sexes during different breeding stages (nest-building and egg-laying), and with different modes of song playback (paused and continuous). We found that aggressiveness was repeatable in both sexes. Male blue tits showed greater aggressiveness towards male intruders than female intruders, and their aggressiveness levels decreased in the egg-laying stage compared to the nest-building stage. In contrast, we found no significant differences in female aggressiveness between female and male intruders or different breeding stages. As for song type, paused song triggered more aggressiveness in each breeding stage in both sexes. Furthermore, focal bird age influenced aggressiveness — first-year breeders showed stronger aggressive responses than older ones, but only in the nest-building stage. Our results show that intruder sex and breeding stage relate to male but not female aggressiveness, suggesting different ecological trade-offs may underlie the observed variation in female and male aggressiveness.