Supplementary material for "Food availability affects parental anti-predator behaviour in red kites"

Abstract Parental investment theory proposes two non-mutually exclusive hypotheses to explain variation in anti-predator behaviour in relation to the age of offspring: the “reproductive value of offspring” hypothesis and the “harm to offspring” hypothesis. The relative importance of the two factors underlying the hypotheses, reproductive value and harm, may change depending on environmental conditions such as food availability. To test the relative importance of the two hypotheses under different food conditions, we conducted a supplementary feeding experiment in red kite (Milvus milvus) breeding pairs and used a live eagle owl (Bubo bubo) as decoy nest predator to trigger anti-predator behaviour. We used time-to-capture in mist nets mounted next to the decoy predator as proxy for mobbing intensity. Under natural food conditions we found a nearly constant mobbing intensity throughout the entire nestling period. However, under food-enhanced conditions mobbing intensity was reduced in parents with young nestlings and increased in parents with old nestlings. These results suggest greater importance of the “reproductive value of offspring” hypothesis in situations of favourable food availability. Moreover, mobbing intensity depended on brood size and weather conditions. The results suggest that parental anti-predator investment increases with the reproductive value of the brood under favourable breeding conditions, but that this pattern is adjusted to the current context, including the vulnerability of the brood and environmental conditions.摘要 亲代投资理论（Parental investment theory）提出了两个非互斥假说，用以阐释反捕食行为随后代年龄变化的变异：分别为“后代繁殖价值”假说（"reproductive value of offspring" hypothesis）与“对后代的伤害”假说（"harm to offspring" hypothesis）。这两个假说背后的核心因素——繁殖价值与伤害程度，其相对重要性可能随食物可获得性等环境条件发生变化。为了检验不同食物条件下两个假说的相对重要性，我们对红鸢（Milvus milvus）的繁殖对开展了补充投喂实验，并以活雕鸮（Bubo bubo）作为模拟巢捕食者，以此触发亲代的反捕食行为。我们将安置于模拟捕食者旁的雾网（mist nets）的捕获时间作为围捕强度的替代指标。在自然食物条件下，整个育雏期的亲代围捕强度几乎保持恒定；而在食物增强条件下，育有低龄雏鸟的亲代围捕强度有所降低，育有高龄雏鸟的亲代围捕强度则有所升高。研究结果表明，在食物资源充足的情境中，“后代繁殖价值”假说的重要性更为突出。此外，围捕强度还受到窝雏数与天气条件的影响。本研究结果显示，在有利的繁殖条件下，亲代的反捕食投资会随窝雏的繁殖价值提升而增加，但该模式会依据当前情境进行调整，涵盖窝雏的易受攻击性以及所处的环境条件。