Recognizing odd smells and ejection of brood parasitic eggs. An experimental test in magpies of a novel defensive trait against brood parasitism

Abstract

One of the most important defensive host traits against brood parasitism is the detection and ejection of parasitic eggs from their nests. Here, we explore the possible role of olfaction in this defensive behaviour. We performed egg-recognition tests in magpie Pica pica nests with model eggs resembling those of parasitic great spotted cuckoos Clamator glandarius. In one of the experiment, experimental model eggs were exposed to strong or moderate smell of tobacco smoke, whereas those of a third group (control) were cleaned with disinfecting wipes and kept in boxes containing odourless cotton. Results showed that model eggs with strong tobacco scent were more frequently ejected compared with control ones. In another experiment, models were smeared with scents from cloacal wash from magpies (control), cloacal wash or uropygial secretions from cuckoos, or human scents. This experiment resulted in a statistically significant effect of treatment in unparasitized magpie nests in which control model eggs handled by humans were more often rejected. These results provide the first evidence that hosts of brood parasites use their olfactory ability to detect and eject foreign eggs from their nests. These findings may have important consequences for handling procedures of experimental eggs used in egg-recognition tests, in addition to our understanding of interactions between brood parasites and their hosts.

The Odour of Sex: Sex-Related Differences in Volatile Compound Composition among Barn Swallow Eggs Carrying Embryos of Either Sex

“…Indeed, egg odour has been shown to potentially convey a broad spectrum of information, including embryo growth rate and health [68]. Parents have also been shown to accrue information on intra- and inter-specific brood parasitism via odours, suggesting that release of kin-dependent or species-specific odours is a by-product of within- and among-species genetic differences that parents exploit in order to reduce the costs of brood parasitism [14]. Here we showed that, consistent with previous findings [26], parents can also acquire information on sex composition of their clutch in the pre-hatching period.…”

“…Odours are relevant to the way in which birds interact with their social environment and with other species, by mediating individual and kin recognition [12,13] and interactions with their brood parasites [14]. In addition, olfaction can mediate major behavioural functions such as navigation [15], location of food [16] and identification of the own nest [17].…”

“…Increasing evidence on olfactory discrimination ability of birds discloses the possibility that odour also plays a role in parent-offspring communication, including interactions in both the pre- and the post- hatching periods [14,23–25]. For example, odour may represent one of the very few cues at parents’ disposal in order to identify offspring sex before hatching [26].…”

The sight of an adult brood parasite near the nest is an insufficient cue for a honeyguide host to reject foreign eggs

“…As punctured eggs typically rot, they may generate olfactory (Soler et al . ) as well as tactile cues for parasitism. Field observations suggest that punctured eggs alone are an insufficient cue to stimulate egg rejection, since Little Bee‐eaters commonly incubate clutches containing heavily punctured and consequently very rotten eggs (Spottiswoode & Colebrook‐Robjent , Spottiswoode & Koorevaar ), but it is possible that in Bee‐eaters this cue is integrated with others (such as the sight of an adult parasite) to trigger rejection, and that this also improves with experience.…”

Egg ejection cost can limit defence strategies against brood parasitism

“…A host’s failure to recognize its own eggs may be the foremost obstacle involved in the evolution of defence strategies such as ejection of a parasite’s eggs (Soler et al, ). The ability of hosts to recognize parasitic eggs is based upon estimates of ejection and/or acceptance rates of such eggs (Aviles & Garamszegi, ; Moskát, Hansson, Barabás, Bártol, & Karcza, ), with further refinements that involve testing the differences between the ejection rates of mimetic and nonmimetic parasite eggs (Spottiswoode & Stevens, ; Vikan et al, ).…”