How to hatch from an egg of great structural strength. A study of the Common Cuckoo

Abstract

Brood parasitism represents a unique mode of avian reproduction that requires a number of adaptations. For example, to reduce chances of puncture ejection of their eggs by small hosts, brood parasites may have been selected for laying eggs of unusually great structural strength. However, great structural strength of eggshells should hinder hatching. The goals of our study were to establish if chicks of the Common Cuckoo Cuculus canorus have more difficulty with hatching out of their strong eggs than chicks of species with eggs of similar size, and whether they possess any mechanisms facilitating hatching. To achieve these goals, we compared hatching pattern and selected body characteristics of chicks of the Common Cuckoo with those of another altricial species with eggs of a similar size, the Great Reed Warbler Acrocephalus arundinaceus. Although the rate of pecking was similar in the two species, the Common Cuckoo chicks started pecking earlier in relation to their emergence and consequently required more time and a greater cumulative number of pecks for breaking open their eggs than did young Great Reed Warblers. The two species also differed with respect to the pattern of opening their shells; in contrast to the warbler chicks, which enlarged the original pip circularly, the cuckoo chicks opened the egg by systematically creating a long narrow slit until they emerged. Finally, our study of hatched young revealed several differences; the Cuckoo hatchlings were significantly heavier, had a longer forearm, and their egg tooth was located significantly farther from the tip of the beak. The edge used for cutting through the shell was also significantly longer than that of hatchling Great Reed Warblers. To conclude, our data suggest that hatching is more difficult for a Cuckoo than for a Great Reed Warbler and that Cuckoos possess several mechanisms to overcome the problems of hatching from a structurally strong egg.

Cuckoos, cowbirds and hosts: adaptations, trade-offs and constraints

“…Honza et al (2001) showed that Common Cuckoo chicks need more time and effort to crack their egg shell compared to host chicks.…”

Why do brood parasitic birds lay strong-shelled eggs?

“…Therefore, it is possible that cuckoo eggshells may be selectively neutral in terms of egg survival, but important as a source of calcium for a more advanced development of the skeleto-muscular system of the cuckoo chick as an adaptation to its eviction behavior. One study attempted to show that Common Cuckoo hatchlings even possess special morphological adaptations to facilitate hatching from a strong-shelled egg (Honza et al, 2001), indicating that at the time of hatching eggshells are still too strong to be broken open by the hatchling. However, that study compared the morphology of newly hatched cuckoo chicks with that of one of its main passerine hosts, the Great Reed Warbler (Acrocephalus arundinaceus), having similarly sized eggs.…”

“…A necessary pre-condition to favor increased eggshell strength in some parasitic lineages is that their host species should be puncture ejectors, and that puncture ejection should be difficult to accomplish and/or costly. Given that birds are calcium-limited, and that eggshell strength must be traded against the ability of the cuckoo chick to hatch (Honza et al, 2001), it would not pay cuckoos to increase eggshell strength in host species readily able to reject parasitic eggs. Thus, cuckoo eggs are predicted to be thinner-shelled in accepting, grasp ejecting and efficient puncture ejecting host species, and strongershelled in the smallest puncture ejectors since only in the latter case may increase eggshell strength lead to 'forced' acceptance (Antonov et al, 2009).…”

Variation in Plasma Oxidative Status and Testosterone Level in Relation to Egg-Eviction Effort and Age of Brood-Parasitic Common Cuckoo Nestlings

“…For the cuckoo, hatching earlier than the host's young is advantageous as it gives the parasite a competitive advantage (Davies 2000). Cuckoos' rate of embryonic development is generally greater than that of the host's young (Honza et al 2001), although this brood parasite's advanced hatching may also be the result of internal incubation before laying (Birkhead et al 2011). Cuckoo nestlings are able to evict the host's eggs or hatchlings over the rim of the nest while they are still blind and relatively small, typically between 1 and 3 days of age (Wyllie 1981, Davies 2000, Honza et al 2007).…”