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No. 137: SEP-OCT 2001

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Bird's Eggs As Information Carriers

Bird communication may be more subtle than trills and squawks. Take, for example, the observation that the last egg laid by several species is distinctly lighter in color than the others in the clutch. This late-egg phenomenon is seen in the sparrows especially (House, Tree, Dead Sea, Grey-Head, and others). Common Terns, Fieldfares, Herring Gulls, and Moorhens also lay pale last eggs. Since the changes in the egg production line exact a cost in the females, there might be an adaptive explanation for the phenomenon; that is, the final paler egg may lead to increased survival of the participating species. To illustrate, in 1980, Yom-Tov suggested that:

...this last odd egg might have evolved as a signal to potential brood parasites that the female has finished laying the clutch and has begun incubation. If a parasite lays its egg after incubation has commenced, then it would be unlikely to hatch, hence the potential parasite would benefit from heeding such a warning signal, if it could then find an alternative host nest where incubation had yet to commence. The signaling host would also benefit from avoiding the costs of warming an extra egg [the parasite's] for some of the incubation period, costs that can be considerable, as well as the possible cost of having to rear an extra chick.

G.D. Ruxton et al have used game theory to show that Yom-Tov's speculation has considerable merit.

However, there are nonadaptive explanations. The females in the species that lay pale last eggs may simply run out of egg pigment, or they may change physiologically as the egg-laying phase nears its end.

(Ruxton, G.D., et al; "Are Unusually Colored Eggs a Signal to Potential Con-specific Brood Parasites?" American Naturalist, 157:451, 2001.)

Comment. We will not quarrel with game theory but hasten to point out that the evolution of interspecies signalling requires: (1) A population of hosts in which some females, for one reason or other, lay pale last eggs; and (2) A population of parasites in which, for one reason or another, some individuals have an aversion to laying eggs in nests with a pale egg. It takes two to signal!

From Science Frontiers #137, SEP-OCT 2001. 2001 William R. Corliss

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