The buffet opens earlier, but it is no longer as diverse, healthy and plentiful. This is how a study by the Eawag and WSL institutes describes the situation of insectivorous birds in temperate latitudes during brooding. A consequence of global warming.
The offspring of songbirds, such as wrens and swallows, need two to three weeks to grow big and strong enough before they leave the nest. They must receive the greatest amount of food and the best quality possible in this time frame.
On the menu, there are mainly insects, not only terrestrial such as beetles and bees, but also aquatic such as caddisflies or mosquitoes, the Eawag Water Research Institute and the Federal Institute for Water Research said on Tuesday. forest, snow and landscape (WSL) in a joint press release.
Ornithologist Ryan Shipley conducted a study as part of the Blue-Green-Biodiversity research initiative launched by these two institutes. He examined how the abundance of insects and the breeding season of various species of migratory songbirds have changed over 25 years in the northeastern United States in relation to climate change.
At Cornell University in New York state, insects were counted, measured and classified day after day from 1989 to 2014. not only how the total number of insects has changed over 25 years, but also the changes in terms of body mass, diversity and the presence of certain species’.
The researcher has selected seven species of small birds that breed at various times. Climate data shows, as with us, a rise in temperatures in early spring. The emergence of insects takes place earlier, as does the blooming period. Aquatic insects emerge on average a week earlier and terrestrial insects almost two weeks earlier than in the 1990s.
For aquatic insects, the data further shows that their numbers increase much faster in April than before but decrease drastically in May. The period when the supply of aquatic insects is rich has therefore shortened and advanced in the breeding season, according to this work published in the journal Current Biology.
During the second half of the breeding season, terrestrial insects are dominant, their numbers increasing continuously until mid-July, although to a lesser extent than in the past.
While birds that breed early in the season find mostly aquatic insects, those that start breeding in mid-May are left with only terrestrial insects.
Omega-3 on the menu
However, not all insects are created equal: aquatic insects are of better quality than terrestrial insects. Their Omega-3 fatty acid content is several times higher. “Childlings that ingest more of these valuable fats grow faster and can leave the nest sooner,” says Ryan Shipley.
It seems that birds that lay early, like the eastern bluebird, take advantage of this new situation, as they find more aquatic insects. In the region studied, their population has increased since 1966. On the other hand, the number of tree swallows, which brood later and must feed almost exclusively on terrestrial insects, has fallen sharply.
One might expect the birds to adapt to the new insect schedule and start brooding earlier. ‘That is indeed what is happening. However, not to the same extent as the staggered presence of insects in the season,” says Ryan Shipley. In 25 years, the brooding period of observed bird species has advanced from 3 to 7 days.
Unlike many parts of the United States and Europe, the total number of insects in the study region remained stable between 1989 and 2014. The fact that the situation worsened for some birds despite this shows how the timing and the quality of the food source are decisive during the brooding period, concludes Eawag.
Ryan Shipley now intends to conduct a large-scale study with data from Europe, Japan and Russia.