Cuculus canorus

This is the second post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2018. This particular post was written by Simone Dutt and it was titled "Keeping eggs warm: brood parasites and their early-hatching thick-shelled eggs" about a rather different type of parasites to the ones usually featured on this blog - the cuckoo (you can also read a previous post about how parasitoid larvae are affected by what their caterpillar host eats here).

Some birds manage to evade the burdensome task of caring for their own young by laying their eggs in the nests of other birds for them to raise. Cowbirds, honeyguides and the more well-known cuckoos are families of birds in which some members have adopted this parasitic lifestyle. Known as brood parasites, they force unsuspecting host birds to care for the parasitic chicks, often at the expense of their own young.

Photo of newly-hatched Cuculus canorus chick by Per Harald Olsen

Previous studies have found that parasitic chicks tend to hatch earlier than the host’s chicks and that brood parasites also tend to lay eggs with shells that are structurally stronger and thicker than those of both their non-parasitic relatives and their hosts. Hatching early gives a parasitic chick a head-start to enacting their instinctual plans for total nest domination by out-competing their nestmates for food and space, and in some cases, evicting rival eggs from the nest altogether. It is obvious how such extreme sibling rivalry would suit a parasitic lifestyle, however the evolutionary advantages of producing thicker eggshells are not as clear.

Several suggestions have been made to explain this adaptation: to provide extra calcium for chicks that require stronger bodies with which to evict their nestmates; to provide protection from microorganisms; or to prevent damage incurred during laying, incubation or being punctured and evicted by a host. While these benefits certainly apply to some brood parasites, they don’t generally apply to all. For example, some parasitic chicks can play nicely and refrain from evicting their step-siblings, and many of the dangers faced by parasitic eggs also affect non-parasitic eggs, thus these explanations are not wholly adequate.

A recent study published in The Science of Nature offers evidence to support a more general explanation for the evolution of early-hatching eggs and thicker eggshells in brood parasites. Based on the well-established knowledge that elevated incubation temperatures improve the development rate of bird (and other egg-borne) embryos, the researchers hypothesised that the brood parasites’ thicker eggshells may have evolved as a kind of insulation to maintain high egg temperatures and improve resistance to temperature disturbances, increasing the embryo development rate and enabling the parasite chicks to hatch earlier and carry out their nest takeover plans unchallenged.

Comparing host and parasite eggs collected from the nests of Oriental reed warblers (Acrocephalus orientalis) parasitised by common cuckoos (Cuculus canorus), they found that the cuckoo eggshells were 17% thicker than the warbler eggshells and that, as expected, the cuckoo chicks all hatched before the warbler chicks. By incubating the eggs in a laboratory and measuring the temperature of the eggshells under different conditions, the researchers found that the cuckoo eggs were significantly warmer than the warbler eggs during normal incubation.

When the incubation temperature was disturbed by exposing the eggs to different-length bouts of cooling, they found that the temperature of the cuckoo eggs remained significantly more stable than the warbler eggs throughout the temperature disturbances. The researchers also found that the warbler eggs exposed to longer periods of cooling required a significantly longer total incubation time before they hatched, whereas the total incubation time for the cuckoo eggs was not significantly affected by the length of cooling bouts.

The researchers suggest that these findings provide a general explanation for the evolutionary drivers of the fast-hatching, thick-shelled eggs of brood parasites and noted that the parasitic eggs also tend to be more spherical than those of their hosts, potentially contributing to their heat-retaining qualities; a possible direction for future study. Evolutionarily speaking, being able to maintain an optimally high temperature and withstand longer periods of cooling is a useful trait for ensuring parasitic chicks maintain their ability to hatch early in the nests of a number of different host species, who may leave their eggs unattended for varying periods of time during incubation.

Reference:
Yang, C., Huang, Q., Wang, L., Du, W.-G., Liang, W., & Møller, A. P. (2018). Keeping eggs warm: thermal and developmental advantages for parasitic cuckoos of laying unusually thick-shelled eggs. The Science of Nature, 105:10

This post was written by Simone Dutt