Birds are laying their eggs earlier and climate change is to blame, according to study

Nests from the 19th century show how global warming has brought the breeding season forward by almost a month compared to 100 years ago

Chicago’s Field Museum holds thousands of eggs collected since 1872 with records on which species laid them and when.
Chicago’s Field Museum holds thousands of eggs collected since 1872 with records on which species laid them and when.Bill Strausberger

At the end of the 19th century, it became fashionable among bird lovers in Central Europe, as well as the British Isles and the United States, to search for birds’ nests and steal their eggs. Now such a practice would be unthinkable, not to mention forbidden in many countries. But that frowned-upon hobby has allowed a group of researchers to compare the month when the eggs were laid back then with when they are laid today and determine that birds now nest almost a month earlier than a century ago; and when it comes to apportioning blame, the recently published research indicates that climate change is the most likely culprit.

Global warming is causing a myriad of natural phenomena: trees are sprouting earlier, insects are starting to fly sooner and birds are migrating earlier. All this is affecting the breeding season, one of the most important events in the life cycle of any species. But to what extent? To answer that question we need databases that go back a significant number of years.

Researchers at the Field Museum of Natural History in Chicago have been able to go as far back as 1872, when the United States was still recovering from the Civil War. The museum has thousands of bird eggs that were collected between the tail-end of the 19th century and the first third of the 20th, after which raiding nests went out of fashion; the custom was so widespread that the authorities had to issue bans to save a number of species from extinction.

Ornithologist John Bates, the curator of the museum, stresses the thoroughness of the collectors: “They were very good at filling out data sheets with detailed information about when and where the eggs were found, including incubation times,” he says. “Using this information and egg-laying biology [basically, once it starts, the female lays another egg every day], you can accurately calculate the date the first egg was laid.”

Bates and ornithologists from other institutions have used these records to study whether the laying period has come forward, as feared. To do so, they compared dates registered more than a century ago with those recorded for their own purposes since 1980. Their results, published in the Journal of Animal Ecology, reveal that of the 72 species for which there are comparable past and present records, egg-laying now occurs an average of 25.1 days earlier among one-third of the birds. But there are some birds that have brought laying forward by as much as 50 days while only one species, the American blackbird, nests later.

Earlier egg-laying is not limited to the American Midwest. In fact, it is becoming a widespread phenomenon, at least in the northern hemisphere. Studies in Finland, the Netherlands and the United Kingdom also show that birds are expanding their families earlier in the year. The British Trust for Ornithology is the leading bird-conservation institution in the United Kingdom and its egg collection dates back to 1929. A 2020 report noted that 38 species of birds common to Britain had brought egg-laying forward by between three and 21 days since 1960.

While pied flycatchers had brought forward their laying schedule by three days, the caterpillars on which they fed had done so by 15 days
Ornithologist Óscar Gordo

In Spain, the National Museum of Natural Science (MNCN) conserves eggs collected in the 18th century, but they have not yet been studied in depth. Juan José Sanz is an ornithologist at the MNCN and in his opinion, “the results of this research in the United States are part of a general pattern, but they have been able to go way back.” The US research links the earlier laying dates to an increase in the concentration of carbon dioxide in the atmosphere and, therefore, a rise in temperatures. “All species are very dependent on temperature,” Sanz points out. In the case of birds, they are dependent on external climatic cues to let them know when to make or search for a nest. “The gonads of male birds, for example, remain retracted during the winter and only become active as the weather improves,” adds Sanz.

Ornithologist Óscar Gordo argues that the change in laying dates “does not tell us anything in itself; what matters is its relationship with other elements in the ecosystem.” The key elements are temperature and food availability. According to Bates, “most of the birds we have studied eat insects and the insects’ seasonal behavior is also affected by climate,” referring to the fact that climate change has also altered the onset of insect flight.

The danger is that the mechanism that has been fine-tuned over thousands and thousands of years may get out of sync. Gordo flags up the case of a pioneering study by a group of Dutch ornithologists who found some years ago that, “while the pairs of pied flycatchers had brought forward their laying schedule by three days, the caterpillars on which they fed had done so by 15 days.”

Every year since 1988, Jaime Potti has been going to two forests in the north of Madrid to observe the nests of the local flycatcher populations. He is now retired as a scientist at Spain’s CSIC research center, but he still goes up to the mountains every spring to study the little black and white male bird and gray female. “In the 1990s, it was hot,” says Potti. “We went in our shirtsleeves and we noticed that they had come back [from migration] earlier.”

This species spends winters in central Africa, in the Gulf of Guinea, and returns in April along the coast of the continent. “But in the following decade, we would come up dressed warmly because it was cold, and the flycatchers would return later,” he adds. According to Potti, the birds show great flexibility when it comes to the temperature variable that most dictates when they lay their eggs.

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