Embryo movement is more frequent in avian brood parasites than birds with parental reproductive strategies

Extra Nestlings That Are Condemned to Die Increase Reproductive Success in Hoopoes

AbstractThe adaptive value of routinely laying more eggs than can be successfully fledged has intrigued evolutionary biologists for decades. Extra eggs could, for instance, be adaptive as insurance against hatching failures. Moreover, because recent literature demonstrates that sibling cannibalism is frequent in the Eurasian hoopoe (Upupa epops), producing extra offspring that may be cannibalized by older siblings might also be adaptive in birds. Here, directed to explore this possibility in hoopoes, we performed a food supplementation experiment during the laying period and a clutch size manipulation during the hatching stage. We found that females with the food supplement laid on average one more egg than control females and that the addition of a close-to-hatch egg at the end of the hatching period increased the intensity of sibling cannibalism and enhanced fledging success in hoopoe nests. Because none of the extra nestlings from the experimental extra eggs survived until fledging, these results strongly suggest that hoopoes obtain fitness advantages by using temporarily abundant resources to produce additional nestlings that will be cannibalized. These results therefore suppose the first experimental demonstration of the nutritive adaptive function of laying extra eggs in vertebrates with parental care.

Eggshell composition and surface properties of avian brood-parasitic species compared with non-parasitic species

The eggs of avian obligate brood-parasitic species have multiple adaptations to deceive hosts and optimize development in host nests. While the structure and composition of the eggshell in all birds is essential for embryo growth and protection from external threats, parasitic eggs may face specific challenges such as high microbial loads, rapid laying and ejection by the host parents. We set out to assess whether eggshells of avian brood-parasitic species have either (i) specialized structural properties, to meet the demands of a brood-parasitic strategy or (ii) similar structural properties to eggs of their hosts, due to the similar nest environment. We measured the surface topography (roughness), wettability (how well surfaces repel water) and calcium content of eggshells of a phylogenetically and geographically diverse range of brood-parasitic species (representing four of the seven independent lineages of avian brood-parasitic species), their hosts and close relatives of the parasites. These components of the eggshell structure have been demonstrated previously to influence such factors as the risk of microbial infection and overall shell strength. Within a phylogenetically controlled framework, we found no overall significant differences in eggshell roughness, wettability and calcium content between (i) parasitic and non-parasitic species, or (ii) parasitic species and their hosts. Both the wettability and calcium content of the eggs from brood-parasitic species were not more similar to those of their hosts' eggs than expected by chance. By contrast, the mean surface roughness of the eggs of brood-parasitic species was more similar to that of their hosts' eggs than expected by chance, suggesting brood-parasitic species may have evolved to lay eggs that match the host nest environment for this trait. The lack of significant overall differences between parasitic and non-parasitic species, including hosts, in the traits we measured, suggests that phylogenetic signal, as well as general adaptations to the nest environment and for embryo development, outweigh any influence of a parasitic lifestyle on these eggshell properties.

Visual complexity of egg patterns predicts egg rejection according to Weber's law

Visual complexity is ubiquitous in nature. Drivers of complexity include selection in coevolutionary arms races between antagonists. However, the causes and consequences of biological complexity and its perception are largely understudied, partly because complexity is difficult to quantify. Here, we address this by studying egg pattern complexity and its perception in hosts (tawny-flanked prinia Prinia subflava), which visually recognize and reject mimetic eggs of their virulent brood parasite (cuckoo finch Anomalospiza imberbis). Using field data and an optimization algorithm, we compute a complexity metric which predicts rejection of experimentally placed conspecific eggs in prinia nests. Real cuckoo finch eggs exhibit significantly lower pattern complexity than prinia eggs, suggesting that high complexity benefits hosts because it distinguishes host eggs from parasitic eggs. We show that prinias perceive complexity differences according to Weber's law of proportional processing (i.e. relative, rather than absolute, differences between stimuli are processed in discrimination, such that two eggs with simple patterns are more easily discriminable than two with complex patterns). This may influence coevolutionary trajectories of hosts and parasites. The new methods presented for quantifying complexity and its perception can help us to understand selection pressures driving the evolution of complexity and its consequences for species interactions.

Thicker eggshells are not predicted by host egg ejection behaviour in four species of Australian cuckoo

Defences of hosts against brood parasitic cuckoos include detection and ejection of cuckoo eggs from the nest. Ejection behaviour often involves puncturing the cuckoo egg, which is predicted to drive the evolution of thicker eggshells in cuckoos that parasitise such hosts. Here we test this prediction in four Australian cuckoo species and their hosts, using Hall-effect magnetic-inference to directly estimate eggshell thickness in parasitised clutches. In Australia, hosts that build cup-shaped nests are generally adept at ejecting cuckoo eggs, whereas hosts that build dome-shaped nests mostly accept foreign eggs. We analysed two datasets: a small sample of hosts with known egg ejection rates and a broader sample of hosts where egg ejection behaviour was inferred based on nest type (dome or cup). Contrary to predictions, cuckoos that exploit dome-nesting hosts (acceptor hosts) had significantly thicker eggshells relative to their hosts than cuckoos that exploit cup-nesting hosts (ejector hosts). No difference in eggshell thicknesses was observed in the smaller sample of hosts with known egg ejection rates, probably due to lack of power. Overall cuckoo eggshell thickness did not deviate from the expected avian relationship between eggshell thickness and egg length estimated from 74 bird species. Our results do not support the hypothesis that thicker eggshells have evolved in response to host ejection behaviour in Australian cuckoos, but are consistent with the hypothesis that thicker eggshells have evolved to reduce the risk of breakage when eggs are dropped into dome nests.