Climate change is disrupting bird diets

red-throated warbler

The American Bluebird (Sialia sialis) benefits from earlier fledging of aquatic insects. © Steve Byland, istock

In order to raise their chicks, birds depend on insects being available for their food supply at a given time of year. Researchers have now investigated the consequences of the shifting of the seasons due to climate change for the activity of foraging insects and, in particular, aquatic insects. It turned out that the insect buffet opened earlier, is no longer as varied and only half as full.

The young of small birds such as wrens or barn swallows need two to three weeks until they are big and strong enough to leave the nest. This period of time determines whether the rearing is successful and whether the parent birds can successfully pass on their genes. In this time window, they must therefore be able to procure as much high-quality feed as possible for their offspring. On the menu are not only land insects such as beetles, bees and Co., but also aquatic insects such as caddis flies or mosquitoes. However, it has been known for some time that climate change is affecting the activity of insects. What about the food supply for the birds?

Premature appearance of insects shifts the menu

Ornithologist Ryan Shipley from the Swiss aquatic research institute Eawag and his colleagues have now investigated this question. In their study, they examined how insect occurrence and the breeding season of various songbird and migratory species in the Northeast of the USA have changed over the past 25 years and what role climate change plays in this. In this region, US scientists counted, measured and categorized insects every day from 1989 to 2014. “This data set is unique,” says Shipley. Long-term observation series were also available for the breeding habits and successes of various bird species in the region. For his study, Shipley chose seven species of small birds that breed at different times and feed their young with insects.

The climate data of the study region show, similar to ours, an increase in temperatures in early spring. As was to be expected, insect development also shifted forward with the growing season. On average, aquatic insects appeared about a week earlier in spring and terrestrial insects almost two weeks earlier than in the 1990s. “For aquatic insects, the data also show that the period of abundant supply has shortened and moved forward in the breeding season,” Shipley and his colleagues report. Land insects therefore dominate in the second half of the breeding season, which is crucial for the birds. While early breeders will still find enough aquatic insects, bird species that only start breeding from mid-May are almost exclusively terrestrial insects.

The superfood of birds

This can become a problem for the birds, because food is not just food: aquatic insects are of higher quality than terrestrial insects, they are, so to speak, the superfood of many birds. Among other things, their content of omega-3 fatty acids is many times higher than that of terrestrial insects. “Baby birds who get more of these valuable fats are more likely to grow faster and be able to leave the nest sooner, which is a survival advantage,” explains Shipley. Early breeders like the Red-throated Bluebird benefit from the situation because they find more aquatic insects during the breeding season. However, the number of marsh martins that later breed and now have to make do with terrestrial insects because the seasons have been brought forward has fallen sharply.

One might expect the birds to adapt to the insects’ new schedule and start breeding earlier. “It actually happens. But not to the same extent that insect emergence has accelerated,” says Shipley. In the 25 years, the breeding season of the bird species examined has only shifted forward by three to seven days.

The timing and quality of the buffet are crucial

What surprised the research team: Unlike in many regions of the USA and Europe, the total number of insects in the study area remained stable from 1989 to 2014. They see a possible explanation in the fact that there is comparatively little intensive land use by humans in this area and the habitats are therefore still relatively intact. However, despite stable insect biomass, the situation for some bird species has deteriorated. , This shows how crucial the timing and, in particular, the quality of the food supply is during the breeding season. “In other regions that are severely affected by insect die-offs, shifting and changing food supply would put the birds under even more pressure,” Shipley and his colleagues explain.

Source: Eawag – Swiss Federal Institute of Aquatic Science and Technology; Article: Current Biology, doi: 10.1016/j.cub.2022.01.057

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