probing-limits-egg-recognition-using-egg-rejection-experiments-along

Probing the Limits of Egg Recognition Using Egg Rejection Experiments Along Phenotypic Gradients

Brood parasites lay their eggs in other females' nests, leaving the host parents to hatch and rear their young. Studying how brood parasites manipulate ...

University of Illinois

Obligate avian brood parasitic species impose the costs of incubating foreign eggs and raising young upon their unrelated hosts. The most common host defence is the rejection of parasitic eggs from the nest. Both egg colours and spot patterns influence egg rejection decisions in many host species, yet no studies have explicitly examined the role of variation in spot coloration. We studied the American robin Turdus migratorius, a blue-green unspotted egg-laying host of the brown-headed cowbird Molothrus ater, a brood parasite that lays non-mimetic spotted eggs. We examined host responses to model eggs with variable spot coloration against a constant robin-mimetic ground colour to identify patterns of rejection associated with perceived contrast between spot and ground colours. By using avian visual modelling, we found that robins were more likely to reject eggs whose spots had greater chromatic (hue) but not achromatic (brightness) contrast. Therefore, egg rejection decision rules in the American robin may depend on the colour contrast between parasite eggshell spot and host ground coloration. Our study also suggests that egg recognition in relation to spot coloration, like ground colour recognition, is tuned to the natural variation of avian eggshell spot colours but not to unnatural spot colours.

Does contrast between eggshell ground and spot coloration affect egg rejection?

Oviparous animals have evolved multiple defenses to prevent microbes from penetrating their eggs and causing embryo mortality. In birds, egg constituents such as lysozyme and antibodies defend against microbial infestation, but eggshell pigments might also impact survival of bacteria. If so, microbes could exert an important selective pressure on the evolution of eggshell coloration. In a previous lab experiment, eggshell protoporphyrin caused drastic mortality in cultures of Gram positive, but not Gram negative, bacteria when exposed to light. Here, we test this "photodynamic antimicrobial hypothesis" in a field experiment. In a paired experimental design, we placed sanitized brown, protoporphyrin-rich chicken eggs alongside white eggs that lack protoporphyrin. We deployed eggs for 48 hr without incubation, as can occur between laying and incubation, when microbial infection risk is highest. Eggs were placed on the open ground exposed to sunlight and in dark underground storm-petrel burrows. We predicted that the proportion of Gram-positive bacteria on brown eggs should be lower when exposed to sunlight than when kept in the dark, but we expected no such difference for white eggs. Although our data revealed variation in bacterial community composition, the proportion of Gram-positive bacteria on eggshells did not vary by egg color, and there was no interaction between egg color and location. Instead, Gram-positive bacteria were proportionally more common on eggs on the ground than eggs in burrows. Overall, our experiment did not support the photodynamic antimicrobial hypothesis. The diverse range of avian egg colors is generated by just two pigments, but over 10 hypotheses have been proposed for the evolution of eggshell color. If our results are generalizable, eggshell protoporphyrin might not play a substantial role in defending eggs against microbes, which narrows the field of candidate hypotheses for the evolution of avian eggshell coloration.

Eggshells as hosts of bacterial communities: An experimental test of the antimicrobial egg coloration hypothesis.

Hosts of avian brood parasites often use visual cues to reject foreign eggs, and several lineages of brood parasites have evolved mimetic eggshell coloration and patterning to circumvent host recognition. What is the mechanism of parasitic egg color mimicry at the chemical level? Mimetic egg coloration by Common Cuckoos Cuculus canorus is achieved by depositing similar concentrations of colorful pigments into their shells as their hosts. The mechanism of parasitic egg color mimicry at the chemical level in other lineages of brood parasites remains unexplored. Here we report on the chemical basis of egg color mimicry in an evolutionarily independent, and poorly studied, host-parasite system: the Neotropical Striped Cuckoo Tapera naevia and one of its hosts, the Rufous-and-white Wren Thryophilus rufalbus. In most of South America, Striped Cuckoos lay white eggs that are identical to those of local host species. In Central America, however, Striped Cuckoos lay blue eggs that match those of the Rufous-and-white Wren, suggesting that blue egg color in these cuckoo populations is an adaptation to mimic host egg appearance. Here we confirm that Striped Cuckoo eggs are spectrally similar to those of their hosts and consistently contain the same major eggshell pigment, biliverdin. However, wren eggshells lacked protoporphyrin, which was present in the parasitic cuckoo eggshells. Furthermore, biliverdin concentrations were significantly lower in cuckoo eggshells than in host eggshells. Similarity of host-parasite eggshell appearance, therefore, need not always be paralleled by a quantitative chemical match to generate effective visual mimicry in birds.

Miri Dainson

Department of Animal Biology,
School of Integrative Biology,
Department of Animal Biology, School of Integrative Biology

University of Illinois

Probing the Limits of Egg Recognition Using Egg Rejection Experiments Along Phenotypic Gradients

Miri Dainson

Department of Animal Biology,School of Integrative Biology,Department of Animal Biology, School of Integrative Biology

University of Illinois

Probing the Limits of Egg Recognition Using Egg Rejection Experiments Along Phenotypic Gradients

Department of Animal Biology,
School of Integrative Biology,
Department of Animal Biology, School of Integrative Biology