Credit: ROY HARVEY

There is a happy history of long-term data collected by volunteer ornithologists being used to tackle ecological questions. The latest example is described elsewhere in this issue (Nature 412, 436–438; 2001) by Jennifer Gill and colleagues. Their subject is the black-tailed godwit, Limosa limosa (pictured), a large migratory wader.

Godwits that breed in western Europe, including Britain, mostly winter in Africa. But birds from an Icelandic population winter around the British coast, returning to Iceland each spring to breed. For reasons that are unclear, this Icelandic population has increased steadily for several decades, resulting in more winter migrants to Britain.

Combining census data and new surveys, Gill et al. find that British estuaries vary in quality as godwit habitats. Bird numbers at the best estuaries have remained relatively stable, and the overall population increase has been buffered by a disproportionate increase in birds using lower-quality sites. This cannot be explained by improved habitat quality, as prey availability does not seem to have changed. Gill et al. find that the food-intake rate of godwits at poorer sites is lower than on the traditionally favoured estuaries. Consequently, these birds have higher winter mortality; and those that do survive reach the Icelandic breeding grounds later, perhaps missing out on the best breeding territories.

Lower probability of survival or reproductive success could be a mechanism for population regulation. When the total population density is low, high-quality sites will be favoured. But as their numbers increase, some birds must use lower-quality habitats where they fare less well. Such density-dependent regulation could eventually check the current population increase.