Apr. 25, 2013 — Songbird populations can handle far more disrupting climate change than expected. Density-dependent processes are buying them time for their battle. But without (slow) evolutionary rescue it will not save them in the end, says an international team of scientists led by the Netherlands Institute of Ecology (NIOO-KNAW) in Science this week.
Yes, spring started late this year in North-western Europe. But the general trend of the four last decades is still a rapidly advancing spring. The seasonal timing of trees and insects advance too, but songbirds like Parus major, or the great tit, lag behind. Yet without an accompanying decline in population numbers, it seems, as the international research team shows for the great tit population in the Dutch National Park the Hoge Veluwe.
"It's a real paradox," explain Dr Tom Reed and Prof Marcel Visser of the Netherlands Institute of Ecology. "Due to the changing climate of the past decades the egg laying dates of Parus major have become increasingly mismatched with the timing of the main food source for its chicks: caterpillars. The seasonal timing of the food peak has advanced over twice as fast as that of the birds and the reproductive output is reduced. Still, the population numbers do not go down." On the short term, that is, as Reed, Visser and colleagues from Norway, the USA, and France have now calculated using almost 40 years of data from this songbird.
The solution to the paradox is that although fewer offspring now fledge due to food shortage, each of these chicks has a higher chance of survival until the next breeding season. "We call this relaxed competition, as there are fewer fledglings to compete with," first author Reed points out. Out of 10 eggs laid, 9 chicks are born, 7 fledge and on average only one chick survives winter. That last number increases with less competitors around.