Introduction
In many animals, parental care involves not only provisioning food and protection from inclement weather, but also active nest defence against predators (Clutton-Brock 1991). Parents may prevent nest predation through mobbing, i.e. behaviours such as approaching and harassing the predator, or through loud vocalization and physical attacks (Caro 2005). The resulting departure of a predator from the nest area has a direct benefit (Curio 1978) for offspring survival (Ajie et al. 2007; Andersson et al. 1980; Lind and Cresswell 2005; Montgomerie and Weatherhead 1988), but driving off a predator is costly in terms of time and energy for the parents (Dugatkin and Godin 1992), and includes the risk of being injured or killed by the predator (King 1999; Mo 2017; Sordahl 1990). Hence, mobbing represents a form of parental investment, and according to life history theory, the level of this investment should depend on its cost-benefit-ratio for the parents (Clutton-Brock 1991). Thus, it is expected that mobbing behaviour varies in relation to a number of internal and external factors associated with the benefits of parental efforts (Caro 2005; Mahr et al. 2015).
Parental investment theory proposes two non-mutually exclusive hypotheses to explain intraspecific variation in mobbing intensity and risk-taking during reproduction (see Dale et al. 1996; Swaisgood et al. 2003). The reasoning underlying the two hypotheses results in contrasting predictions. According to the “reproductive value of offspring” hypothesis, there should be a positive relationship between anti-predator investment and the value of the brood (i.e., large brood size, old offspring, good body condition) due to the increase in expected benefits associated with brood survival (Greig-Smith 1980; Montgomerie and Weatherhead 1988; Tryjanowski and Golawski 2004). On the other hand, the “harm to offspring” hypothesis (also called “offspring vulnerability” hypothesis) assumes that parents should increase their anti-predator investment based on the harm the nestlings would suffer from a period of no parental care (i.e., in vulnerable situations where continued care is required). According to this hypothesis, the benefits of the parental investment are generally expected to be highest for broods in poor condition or broods that are unable to manage on their own (i.e. in young offspring; Dale et al. 1996; Listoen 2000). Previous studies investigated these hypotheses by quantifying age-dependent mobbing intensity or risk-taking; however, results are equivocal, providing support for either, or both hypotheses (Crisologo et al. 2017; Fernandez and Llambías 2013; Tryjanowski and Golawski 2004). The reason for this might be that the age-dependent benefits of mobbing behaviour in the two hypotheses may change with breeding conditions, while the age-dependent costs remain similar (for theoretical details, see Dale et al. 1996). While adjustments to mobbing behaviour and risk-taking with different predators or different levels of predation risk were supported in many studies (Carlson et al. 2017; Curio 1983; Dassow et al. 2012; Mahr et al. 2015; e.g. Patterson et al. 1980), only few empirical tests exist examining the effect of breeding conditions on the relative importance of the two hypotheses (but see Hakkarainen and Korpimäki 1994; Listoen 2000).
In many birds, including raptors, high food availability is associated with a high offspring survival rate and reduced harm during periods of parental absence (Fuller 2012; Grüebler et al. 2018; Martin 1987; Nägeli et al. 2022; Perrig et al. 2014; Wellicome et al. 2013). Under such conditions, the importance of the brood’s reproductive value for parental investment decision-making is high, and parents should invest more into protecting old than young broods (Dale et al. 1996). In contrast, under poor food conditions, the deciding factor may shift towards the harm offspring would suffer if they received no care. Thus, under poor food conditions, parents should invest more into young broods than they would under favourable food conditions, and more into young than old broods (Dale et al. 1996; Listoen 2000). Manipulating food availability through supplementary feeding experiments could, therefore, be a suitable approach to test these predictions of food condition-dependent mobbing behaviour in birds.
Techniques to catch adult raptors often take advantage of their anti-predator behaviours. A stuffed or live predator as decoy is presented near the nest to trigger anti-predator reactions and capture the parents (Bloom et al. 1992; Bloom et al. 2007; Zuberogoitia et al. 2008). During capture events, predator placement therefore can also be used for the investigation of mobbing behaviour (Arroyo et al. 2001; Carlson et al. 2017; Sternalski and Bretagnolle 2010; e.g. Tolonen and Korpimaki 1995). Capturing success and time-to-capture should be associated with the intensity of the mobbing behaviour. However, in this context, the time until detection of the decoy predator represents a source of bias in the temporal patterns of the capturing history. This is particularly an issue, because food availability and supplementary feeding affect foraging and movement behaviour (Catry et al. 2013; Grüebler et al. 2018; Pfeiffer and Meyburg 2015; Staggenborg et al. 2017), potentially affecting predator detection.
Here, we aim at testing the food condition-dependent change in the relevance of the “reproductive value of offspring” and the “harm to offspring” hypotheses in a large raptor species, the red kite (Milvus milvus ). We quantified the intensity of mobbing behaviour in relation to offspring age for pairs provided with supplementary food and those without, while accounting for confounding factors such as brood size, natural food availability, weather conditions, and site characteristics. We used capture probability and time-to-capture when responding to a living eagle owl as a measure of mobbing intensity. We also considered the time to detection of the predator and analysed the factors affecting detection time. This study therefore contributes to a more holistic understanding of parental investment under different environmental conditions.