Predictors of aggressive response towards simulated intruders depend on context and sex in Crimson Finches (Neochmia phaeton)

Quantifying differences in aggressive behaviour across contexts can be useful in developing an understanding of life histories and breeding systems, as well as the relative costs and benefits of such behaviour. We investigated whether age, relative body size and colouration, sex, and breeding stage influenced levels of aggressive behaviour in two contexts, towards conspecific and heterospecific intruders (mounts) around active nests of group living Crimson Finches (Neochmia phaeton). We found that when responding to a conspecific mount, relative body size, and age were important in predicting the aggressive response of males toward a conspecific, with older males and those close in size to their opponent showing a higher aggressive response. On the other hand, factors relating to female aggression were not as clear. In contrast, response to a heterospecific mount was unrelated to age, colour or size in either sex. Additionally, although birds were equally aggressive to conspecific and heterospecific mount types, we found no evidence that individuals are consistent in their level of aggression across these contexts. This suggests that aggressive behaviour in Crimson Finches is at least partially plastic and that individuals may be capable of assessing and responding to situations independently.