At the end of the 19th century it became fashionable among bird lovers in Central Europe, the British Isles or the United States to search for bird nests and take their eggs. Now such a practice would be unthinkable (besides prohibited in many countries). But that misunderstood love has allowed a group of researchers to compare when spawning was then and when it is today: birds nest almost a month earlier than a century ago. And everything indicates, according to his recently published research, that climate change is behind it.
Global warming is causing a myriad of unusual phenomena in nature: the trees sprout earlier, the flight of insects begins earlier, the migration of birds is coming earlier… All of this would be affecting one of the events of greatest impact on the life of any species, the breeding season, but to what extent? To answer that question, we need databases that go way back in time.
Researchers of the Field Museum of Natural History (Chicago, United States) have been able to date back to 1872, when that country was still recovering from the Civil War and Amadeo I of Savoy reigned in Spain. This museum has thousands of bird eggs that were collected between the last third of the 19th century and the first of the 20th, when nest raiding went out of fashion. The custom was so widespread that the authorities had to issue prohibitions because many species were on their way to extinction. Ornithologist John Bates, curator of the bird museum, highlights the thoroughness of those collectors: “They were very good at filling out fact sheets with detailed information about when and where the eggs were found, including incubation time. Using this information and the biology of egg laying (basically, once it starts, the female lays another egg every day), you can accurately calculate the date the first egg was laid.”
“Most of the birds we have studied eat insects and their seasonal behavior is also affected by the weather”
John Bates, curator at the Field Museum in Chicago and lead author of the study
Bates and ornithologists from other institutions have used these records to study whether, as they feared, the spawns have advanced. To do this, they compared the dates obtained more than a century ago with those compiled since 1980 by themselves. Their results, published in the scientific journal Journal of Animal Ecology, reveal that of the 72 species for which there are comparable past and present records, in a third of them egg laying now occurs 25.1 days earlier on average. But there are birds that have advanced it up to 50 days. Only one species, the blackbird, nests later.
The advancement of the setting is not limited to the Midwest of the United States, but is being global, at least in the northern hemisphere. Studies in Finland, the Netherlands or the United Kingdom also show that the birds are anticipating the time to expand the family. The British Trust for Ornithology is the UK’s leading bird conservation institution. His egg collection dates back to 1929. A 2020 report It highlighted that 38 species of birds common to British skies had laid their eggs between three and twenty-one days earlier since 1960.
In Spain, the National Museum of Natural Sciences (MNCN) preserves eggs collected in the 18th century, but they have not been thoroughly studied yet. Juan José Sanz is an ornithologist at the MNCN. “The results of this research in the United States are part of a general pattern, but they have been able to go back a long way,” he says. The American work relates the advance to an increase in the concentration of CO2 in the atmosphere and, therefore, the temperature. “All species are very dependent on temperature,” recalls Sanz. In the case of birds, to external climatic clues to start building or looking for a nest. “The gonads of male birds, for example, remain retracted during the winter and are only activated when the weather improves,” concludes Sanz.
Óscar Gordo, from the Catalan Institute of Ornithology, recalls that the change in spawning days “doesn’t tell us anything in itself, what matters is the relative date, its relationship with other elements of the ecosystem.” And the key elements are temperature and food availability. Bates, the curator of the Field Museum, comments that “most of the birds we’ve studied eat insects, and their seasonal behavior is also affected by climate.” Indeed, climate change has also altered the initiation of insect flight. The danger here is that a misalignment occurs in a mechanism adjusted over thousands and thousands of years. Gordo recalls a pioneering study by a group of Dutch ornithologists who found years ago that “while pied flycatcher pairs had laid their eggs three days earlier, the caterpillars they fed on had done so in 15 days.”
Jaime Potti has been going to two forests in the north of Madrid every year since 1988 to see the nests of the local populations of flycatchers. A few years ago he retired as a CSIC researcher, but he continues to go up to the mountains every spring to study this little bird of black and white (the males) and shades of gray (the females). “In the 90s it was hot, we went in shirt sleeves and we saw that they came back earlier,” recalls Potti. This species spends the winters in central Africa, in the Gulf of Guinea, and returns in April along the coast of the continent. “But in the next decade we were bundled up, it was cold, and the flycatchers would come back later,” he adds. For Potti, the little birds show great plasticity in the face of the variable that most affects when to lay their eggs, temperature.
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