How Climate Change Is Leading To Birds Laying Eggs Earlier Than Normal

Scientists determined that about a third of the bird species nesting in Chicago have moved their egg-laying by an average of 25 days.

Representative Image | Getty Images

New Delhi: Many species of birds are nesting and laying eggs nearly a month earlier than they did a hundred years ago, according to a new study. Scientists compared recent observations with century-old eggs preserved in museum collections, and determined that about a third of the bird species nesting in Chicago have moved their egg-laying by an average of 25 days.

The study, led by researchers at the Field Museum in Chicago, was recently published in the Journal of Animal Ecology.

John Bates, curator of birds at the Field Museum and the study's lead author, said that egg collections are such a fascinating tool for researchers to learn about bird ecology over time, according to a statement issued by the museum. 

The Study Looked At Trends Of Bird Ecology Over About 120 Years

According to Bates, the study combines the older and modern datasets to look at the trends of bird ecology over about 120 years, and help answer critical questions about how climate change is affecting birds.

Bates was editing a book about eggs which made him interested in studying the museum's egg collections. 

The museum's egg collection can be found in a small room crammed full with floor-to-ceiling cabinets, each containing hundreds of eggs. Most of these eggs were collected a century ago. 

The eggs are preserved in small boxes, and marked with hand-written labels, which say what kind of bird they belong to, where they are from, and precisely when they were collected. Just the clean, dry shells, with the contents blown out a hundred years ago, are stored in the museum.

After the 1920s, the Field's egg collection dropped off. This is because egg-collection went out of fashion after the 1920s, both for amateur hobbyists and scientists. 

Bill Strausberger and Chris Welan are two researchers whose contributions to the study were critical, according to Bates. 

Strausberger is a research associate at the Field who had worked for years on cowbird parasitism at the Morton Arboretum, a public garden and outdoor museum in the Chicago suburbs. He spent years climbing ladders and examining nests to see whether brown-headed cowbirds had laid their eggs for other birds ro raise. 

Whelan is an evolutionary ecologist at the University of Illinois in Chicago who contributed to the modern dataset with songbird nesting data collected in Chicagoland starting in 1989. Bates said that finding nests is a lot harder than almost anybody realises. 

Whelan explained that finding nests and following their fate to success or failure is extremely time-consuming and challenging. He said the researchers learnt to recognise what he called 'nesty' behaviour. 

The 'nesty' behaviour includes gathering nest material, like twigs, grass, roots, or bark, depending upon bird species, or capturing food like caterpillars but not consuming the food item, Whelan explained. According to him, capturing food but not consuming it likely indicates a parent is foraging to gather food for nestlings. 

In order to peer into high-up nests and keep a close track of the dates when eggs were laid and hatched, Whelan and his colleagues used mirrors mounted on long poles.

Nesting Data From 1880 To 1920 And 1990 To 2015 Were Considered

The study authors had two big sets of nesting data. One set was from roughly 1880 to 1920, and the other was from about 1990 to 2015. Bates said there is a gap in the middle, and that is where Mason Fidino came in.

Fidino, who co-authored the study, is a quantitative ecologist at Chicago's Lincoln Park Zoo, and built models for analysing the data that allowed the researchers to address the gap in the middle of the 20th century. The models also helped the team address differences in sampling between early egg collectors and Whelan and Strausberger's research.

Birds Were Laying Their First Eggs 25.1 Days Earlier Than They Were A 100 Years Ago

When the researchers analysed the data, they observed a surprising trend. Among the 72 species of birds for which historical and modern data were available in the Chicagoland region, about a third have been nesting earlier and earlier, according to the study.

The birds whose nesting habits changed were laying their first eggs 25.1 days earlier than they were a hundred years ago, the study said.

The researchers not only illustrated that birds are laying eggs earlier, but also looked for the reason behind it. 

Rising Temperatures And Carbon Dioxide Levels Correlated With Changes In Egg-Laying Patterns

The team looked to rising temperatures as a potential explanation for the earlier nesting, considering that the climate crisis has dramatically affected several aspects of biology. 

However, consistent temperature data for the region going back that far is not available. Therefore, the researchers turned to the amount of carbon dioxide in the atmosphere, which is a proxy for temperature.

Bates said the team could not find a single source of long-term temperature data for the Midwest, which was surprising, but one can approximate temperature with carbon dioxide levels, which are very well documented. 

The researchers collected the carbon dioxide data from a variety of sources, including the chemical composition of ice cores from glaciers. 

The team found that the amount of carbon dioxide in the atmosphere over time correlated with the changes in egg-laying dates. 

According to the study, the changes in temperature are seemingly small, just a few degrees. However, the little changes translate to different plants blooming and insects emerging. These are factors which could affect the food available for birds.

Bates said that the majority of the birds they looked at eat insects, and insects' seasonal behaviour is also affected by climate. He explained that the birds have to move their egg-laying dates to adapt.

Significance Of The Study

Bates said that all birds have different evolutionary histories and different breeding biology. He added that the changes in nesting dates might result in the birds competing for food and resources in a way that they did not earlier. 

He said that the study not only serves as a warning about climate change, but also highlights the importance of museum collections, particularly egg collections, which are often under-utilised. He further said that there are five million eggs out there in collections worldwide, and yet, there are very few publications using museum collections of eggs. 

Bates asserted that museum collections of eggs are a treasure trove of data about the past, and can help scientists answer important questions about the world today.