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

Brood parasitism represents a unique mode of avian reproduction that requir es a number of adaptations. For example, to reduce chances of puncture ejec tion of their eggs by small hosts, brood parasites may have been selected f or laying eggs of unusually great structural strength. However, great struc tural 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 wi th eggs of similar size, and whether they possess any mechanisms facilitati ng hatching. To achieve these goals, we compared hatching pattern and selec ted body characteristics of chicks of the Common Cuckoo with those of anoth er altricial species with eggs of a similar size, the Great Reed Warbler Ac rocephalus arundinaceus. Although the rate of pecking was similar in the tw o species, the Common Cuckoo chicks started pecking earlier in relation to their emergence and consequently required more time and a greater cumulativ e number of pecks for breaking open their eggs than did young Great Reed Wa rblers. The two species also differed with respect to the pattern of openin g their shells; in contrast to the warbler chicks, which enlarged the origi nal pip circularly, the cuckoo chicks opened the egg by systematically crea ting a long narrow slit until they emerged. Finally, our study of hatched y oung revealed several differences; the Cuckoo hatchlings were significantly heavier, had a longer forearm, and their egg tooth was located significant ly 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 Warbl ers. To conclude, our data suggest that hatching is more difficult for a Cu ckoo than for a Great Reed Warbler and that Cuckoos possess several mechani sms to overcome the problems of hatching from a structurally strong egg.