Scientists have compared the laying dates of 72 bird species and found that rising temperatures are forcing them to lay their eggs a month earlier than they did a century ago.
Nesting 25 days earlier
The arrival of spring announces each year a traditional revival of nature with the song of birds. The latter take advantage of this period to build their nests and lay their eggs. But the Journal of Animal Ecology has published a latest study showing that many species of birds are nesting and laying eggs a month earlier than a century ago.
This change was found by comparing century-old egg collections kept in museums with recent observations, which made it possible to examine egg-laying trends over approximately 120 years.
With this data, the scientists in this study found that, of the 72 bird species in Chicago — for which historical and modern data were available — about one-third now lay eggs 25.1 days earlier than a hundred years ago.
Understanding the behavioral change of birds
John Bates, first author of the study, and his colleagues tried to understand this change in behavior and looked at the rises in temperature which are already the cause of several environmental and biological problems.
However, the team of researchers found themselves faced with a problem: there is no temperature data going back that far in this place. To fill this gap, they used the quantities of carbon dioxide (CO 2 ) in the atmosphere , which is a good temperature indicator. Indeed, by comparing the tendencies of the quantities of CO 2 and of the temperature, one realizes that their variations correspond .
After comparison, the researchers found that variations in CO 2 were correlated with changes in laying dates. It would therefore seem that warming is causing changes that have consequences on the biological behavior of birds: the flowering of plants occurs earlier and is therefore accompanied by the presence of insects .
Since the birds feed on insects, they are forced to come out earlier to be sure of finding available food and are eventually forced to build their nests and lay their eggs earlier.