Ecological Health Warning - Laying Eggs May Make You Ill!

Abstract from Nature
20th June 1996

A letter to the magazine Nature (381, 565) has provided strong evidence for the theory that laying eggs may be very costly to the female bird. And this cost may have a strong influence on the evolution of clutch size.

A simple experiment involved removing the first two eggs from some Great Tits nesting in the wild in Sweden, and not from others. Ann Oppliger, Philippe Christe and Heinz Richner expected their experimental birds to compensate for the lost eggs by laying more, and they did - on average one more.

When the offspring were 14-days old the females were caught and a tiny blood sample was taken. This was examined for the prevalence of malarial parasites (Plasmodium) which is common in the population. About 20% of the control group were infected but almost half of the experimental group. This supports the theory that there is a trade-off for the females between current egg production and defence against parasites and, therefore, the female's chance of breeding in the future. The male birds showed no differences between control and experimental groups.

So every egg should have a health warning: "Too big a clutch may damage your health."