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.