Does avian conspicuous colouration increase or reduce predation risk?

Animals often announce their unprofitability to predators through conspicuous coloured signals. Here we tested whether the apparently conspicuous colour designs of the four European Coraciiformes and Upupiformes species may have evolved as aposematic signals, or whether instead they imply a cost in terms of predation risk. Because previous studies suggested that these species are unpalatable, we hypothesized that predators could avoid targeting them based on their colours. An experiment was performed where two artificial models of each bird species were exposed simultaneously to raptor predators, one painted so as to resemble the real colour design of these birds, and the other one painted using cryptic colours. Additionally, we used field data on the black kite's diet to compare the selection of these four species to that of other avian prey. Conspicuous models were attacked in equal or higher proportions than their cryptic counterparts, and the attack rate on the four species increased with their respective degree of contrast against natural backgrounds. The analysis of the predator's diet revealed that the two least attacked species were negatively selected in nature despite their abundance. Both conspicuous and cryptic models of one of the studied species (the hoopoe) received fewer attacks than cryptic models of the other three species, suggesting that predators may avoid this species for characteristics other than colour. Globally, our results suggest that the colour of coraciiforms and upupiforms does not function as an aposematic signal that advises predators of their unprofitability, but also that conspicuous colours may increase predation risk in some species, supporting thus the handicap hypothesis.

Raptor nest decorations are a reliable threat against conspecifics.

Individual quality is often signaled by phenotypic flags, such as bright plumage patches in birds. Extended phenotype signals can similarly show quality, but in these cases the signals are external to the individual, often taking the form of objects scavenged from the environment. Through multiple manipulative experiments, we showed that objects used for nest decoration by a territorial raptor, the black kite (Milvus migrans), act as reliable threats to conspecifics, revealing the viability, territory quality, and conflict dominance of the signaler. Our results suggest that animal-built structures may serve as signaling devices much more frequently than currently recognized.

Coping with uncertainty: breeding adjustments to an unpredictable environment in an opportunistic raptor.

No environment is truly constant in time. As a result, animals have evolved multiple adjustments to cope with such fluctuations. However, the allocation of effort to costly activities that imply long-term commitments, such as breeding, may be extremely challenging when future resources change constantly and unpredictably, a context that has received little investigation. To fill this gap, we studied the breeding response by a wetland-dependent raptor, the black kite Milvus migrans, to within and between-years fluctuations in resource availability (inundation levels). The breeding performance of the population was decomposed into reproductive components expressed in a sequence of successive tasks along the breeding cycle (e.g. timing of laying, clutch size, hatching success, brood reduction). Variation in each component was related to resource levels observed at different key dates of the season in order to test whether and when population-level reproduction was adjusted to available resources. Along a 22-year time-series, inundation levels fluctuated unpredictably within and among years, and mostly affected the later components of kites' reproduction, such as hatching success and the incidence of brood reduction, which were the main determinants of the population yearly breeding output. Results were consistent with multiple adjustments to cope with uncertainty. As the season progressed and resources became easier to assess, a bet-hedging waiting strategy based on a conservatively small, invariant and asynchronous clutch gave way to real-time resource-tracking mechanisms mediated by progressive adjustments to current prey availability, so that population-level breeding rates were determined and tuned to resources rather late in the season. Such adjustments were the likely outcome of the interaction between parental tactics and environmental constraints. Behavioural flexibility, such as dietary opportunism, probably promoted further resistance to resource oscillations. Given that all ecosystems show some degree of unpredictability, resource-tracking adjustments, such as the ones depicted here, are likely to be commonplace in most communities.