Egg discrimination along a gradient of natural variation in eggshell coloration

Accurate recognition of salient cues is critical for adaptive responses, but the underlying sensory and cognitive processes are often poorly understood. For example, hosts of avian brood parasites have long been assumed to reject foreign eggs from their nests based on the total degree of dissimilarity in colour to their own eggs, regardless of the foreign eggs' colours. We tested hosts' responses to gradients of natural (blue-green to brown) and artificial (green to purple) egg colours, and demonstrate that hosts base rejection decisions on both the direction and degree of colour dissimilarity along the natural, but not artificial, gradient of egg colours. Hosts rejected brown eggs and accepted blue-green eggs along the natural egg colour gradient, irrespective of the total perceived dissimilarity from their own egg's colour. By contrast, their responses did not vary along the artificial colour gradient. Our results demonstrate that egg recognition is specifically tuned to the natural gradient of avian eggshell colour and suggest a novel decision rule. These results highlight the importance of considering sensory reception and decision rules when studying perception, and illustrate that our understanding of recognition processes benefits from examining natural variation in phenotypes

Egg discrimination along a gradient of natural variation in eggshell coloration

Accurate recognition of salient cues is critical for adaptive responses, but the underlying sensory and cognitive processes are often poorly understood. For example, hosts of avian brood parasites have long been assumed to reject foreign eggs from their nests based on the total degree of dissimilarity in colour to their own eggs, regardless of the foreign eggs’ colours. We tested hosts’ responses to gradients of natural (blue-green to brown) and artificial (green to purple) egg colours, and demonstrate that hosts base rejection decisions on both the direction and degree of colour dissimilarity along the natural, but not artificial, gradient of egg colours. Hosts rejected brown eggs and accepted blue-green eggs along the natural egg colour gradient, irrespective of the total perceived dissimilarity from their own egg’s colour. By contrast, their responses did not vary along the artificial colour gradient. Our results demonstrate that egg recognition is specifically tuned to the natural gradient of avian eggshell colour and suggest a novel decision rule. These results highlight the importance of considering sensory reception and decision rules when studying perception, and illustrate that our understanding of recognition processes benefits from examining natural variation in phenotypes

Are tits really unsuitable hosts for the Common Cuckoo?

Avian brood parasites exploit hosts that have accessible nests and a soft insect diet. Common Cuckoo (Cuculus canorus) hosts were traditionally classified as suitable if both parameters were fulfilled or unsuitable if one, or both, were not. In line with this view, hole-nesting tits (Paridae) have become a text-book example of unsuitable Cuckoo hosts. Our extensive literature search for Cuckoo eggs hatched and chicks raised by hosts revealed 16 Cuckoo nestlings in Great Tit (Parus major) nests, 2 nestlings and 2 fledglings in Blue Tits (Cyanistes caeruleus), and 1 nestling in a Crested Tit (Lophophanes cristatus) nest. Our own data from natural observations and cross-fostering experiments concur with literature data that Great Tits are able to rear Cuckoo chicks to fledging. The natural observations involve the first known cases where a bird species became parasitized as a byproduct of nest usurpation (take-over). Surprisingly, Cuckoo chicks raised by Great Tits grew better than Cuckoo chicks raised by common hosts, even alongside host own chicks. The frequency of Cuckoo parasitism in tits may be underestimated by studying tits in artificial nest-boxes with small entrances that prevent Cuckoos from laying and/or fledging. Results support a view that host suitability is not a categorical parameter (host suitable or unsuitable) but a continuous phenomenon. Understanding the diversity of parameters that determine host selection by Cuckoos is limited, because studies on Cuckoo chick diet, growth, and survival in most hosts are rare. Therefore any data are valuable and provide indispensable material for future meta-analyses

Data from: Rearing a virulent common cuckoo is not extra costly for its only cavity-nesting host

Virulent brood parasites refrain from arduous parental care, often kill host progeny and inflict rearing costs upon their hosts. Quantifying the magnitude of such costs across the whole period of care (from incubation through to parasite fledgling independence) is essential for understanding the selection pressures on hosts to evolve antiparasitic defences. Despite the central importance of such costs for our understanding of co-evolutionary dynamics, they have not yet been comprehensively quantified in any host of any avian brood parasite. We quantified parasite rearing costs in common redstarts Phoenicurus phoenicurus raising either parasitic common cuckoo Cuculus canorus or their own chicks throughout the complete breeding cycle and used multiple cost parameters for each breeding stage: incubation, brooding and feeding effort; length of parental/host care; parent/host body condition and the heterophil/lymphocyte ratio (stress level indicator). Surprisingly and contrary to traditional assumptions, rearing the parasite per se was not associated with overall higher physiological or physical costs to hosts above the natural levels imposed by efforts to rear their own progeny. The low parasite-rearing costs imposed on hosts may in part explain the low levels of known host counter-defences in this unusually frequently parasitised cuckoo host

Additional file 1: of The common redstart as a suitable model to study cuckoo-host coevolution in a unique ecological context

Output of models explaining variation in cuckoo laying date, hatching success and fledging success. (DOCX 24 kb

Samas_et_al_Behav_Ecol_2019_data

Samas_et_al_Behav_Ecol_2019_dat

costs_of_brood_parasitism_data

costs_of_brood_parasitism_dat

Data from: No immediate or future extra costs of raising a virulent brood parasite chick

Parental care is an adaptive behaviour increasing the survival of young. Virulent brood parasites, like the common cuckoo Cuculus canorus, avoid the parental care and leave the care for their nestlings to hosts. Although raising a cuckoo is always costly because it kills host’s progeny, to date it is not known whether raising of a brood parasite itself represents an extra cost affecting host’s fitness. We quantified costs of rearing a cuckoo nestling in the most frequent host, the reed warbler Acrocephalus scirpaceus. We measured changes in the host physical (body mass) and physiological conditions (stress levels quantified via heterophils/lymphocytes ratio) within the one breeding attempt (immediate cost) and re-trapped some of these adults in the next breeding season to estimate return rates as a measure of their survival (future cost). In contrast to universal claims in the literature, raising a cuckoo nestling did not entail any extra immediate or future costs for hosts. This counterintuitive result reconciles theoretical expectations in the hosts with surprisingly low levels of counter-defences, including the reed warbler. Unexpectedly low costs of parasitism may also explain a long-term maintenance of some host-parasite systems