Experimental evidence for chick discrimination without recognition in a brood parasite host

Abstract

Recognition is considered a critical basis for discriminatory behaviours in animals. Theoretically, recognition and discrimination of parasitic chicks are not predicted to evolve in hosts of brood parasitic birds that evict nest-mates. Yet, an earlier study showed that host reed warblers (Acrocephalus scirpaceus) of an evicting parasite, the common cuckoo (Cuculus canorus), can avoid the costs of prolonged care for unrelated young by deserting the cuckoo chick before it fledges. Desertion was not based on specific recognition of the parasite because hosts accept any chick cross-fostered into their nests. Thus, the mechanism of this adaptive host response remains enigmatic. Here, I show experimentally that the cue triggering this 'discrimination without recognition' behaviour is the duration of parental care. Neither the intensity of brood care nor the presence of a single-chick in the nest could explain desertions. Hosts responded similarly to foreign chicks, whether heterospecific or experimental conspecifics. The proposed mechanism of discrimination strikingly differs from those found in other parasite-host systems because hosts do not need an internal recognition template of the parasite's appearance to effectively discriminate. Thus, host defences against parasitic chicks may be based upon mechanisms qualitatively different from those operating against parasitic eggs. I also demonstrate that this discriminatory mechanism is non-costly in terms of recognition errors. Comparative data strongly suggest that parasites cannot counter-evolve any adaptation to mitigate effects of this host defence. These findings have crucial implications for the process and end-result of host-parasite arms races and our understanding of the cognitive basis of discriminatory mechanisms in general.

Is brood parasitism related to host nestling diet and nutrition?

“…Additionally, we used lower-resolution recordings from infrared microcameras (CCD Bird Box Camera 420TVL; SpyCameraCCTV, Bristol, UK) located inside nest boxes; each microcamera was connected to a digital video-recorder (Wireless 2.4 GHz 1 Channel D1 Mini DVR Recorder; SpyCameraCCTV) hidden in an underground box below the nest. Microcameras were primarily used to determine the exact time of fledging of host and parasitic nestlings, as they allowed us to record the brood undisturbed for several days without the need to check the nest in person, which may cause premature fledging (Grim 2007b). However, we also took advantage of these microcamera recordings to determine dietary items, although necessarily at a lower level of taxonomic detail (invertebrate, vertebrate, and plant).…”

“…Video-recordings confirmed that cuckoo nestlings did not try to fledge before we removed the nails. Third, fledging time was primarily determined from longterm microcamera recordings, which started a day or 2 before the expected fledging date for a particular nest and thus eliminated the necessity of repeated nest checks by a human observer during the period shortly before nestlings fledge when human nest checks may trigger premature fledging (see also Grim 2007b).…”

Females that experience threat are better teachers

“…For example, Acrocephalus hosts increase mobbing behaviour at the nest when the risk of parasitism is high [13]. But when brood parasite risk is low, hosts accept more foreign eggs into their nest, thereby lowering the rejection threshold for eggs and chicks [12,14]. Here, we test whether incubating female fairy-wrens increase their incubation call rate when they hear the broadcast of a cuckoo near their nest.…”

Alternative mechanisms of increased eggshell hardness of avian brood parasites relative to host species

“…In turn, cuckoo parasitism severely reduces the host's fitness [2,3], typically because the parasitic chick evicts host eggs and nest-mates [4,5]. This results in coevolutionary arms-races [6] of morphological, physiological and cognitive adaptations and counter-adaptations between parasites and hosts; hosts recognize and avoid parasitism and parasites reduce or circumvent rejection by hosts [1,[7][8][9][10][11][12][13].…”