Healthy Goldcrests Are Better Migrants

Abstract from Functional Ecology
15th April 1996

A study of tiny migrating Goldcrests, at the southern tip of the island of Gotland in the Swedish Baltic, revealed that less-healthy individuals were lighter than healthy birds. They had put on less fat for use as fuel for their migration. 69 young birds were caught between 3 and 9 October 1994. The report by Merila and Svensson appears in Functional Ecology, 9: 842-848.

The birds were weighed, measured, and had a tiny blood sample taken, before being released to continue their movement. The blood sample was analyzed to measure the presence of infection (by sedimentation rate) and anaemia (packed red cell blood volume). The more infected and more anaemic birds were lighter than the less infected and less anaemic ones - which indicated that the latter will be better migrants. Even on such a relatively small sample, the results were highly significant: P<0.001 and P<0.007 respectively.

It is probable that the weaker birds felt no worse than a little 'under the weather' but the effects will have very severe consequences on their efficiency as migrants and their future survival. This is one of the first studies to relate the health of migrant birds to their fitness for migration.

Chris Mead