25 days earlier: global warming makes birds nest sooner
Climate Change Environment Wildlife

25 days earlier: global warming makes birds nest sooner

Dr. Lindy Whitehouse
Dr. Lindy Whitehouse

In 10 seconds? A century-old collection of birds’ eggs has helped scientists determine that birds are nesting earlier than they used to a hundred years ago. The reason? Climate change.

What’s the breakthrough? Researchers have found that a third of the bird species found in Chicago are nesting and laying their eggs a month earlier than they were a hundred years ago. The scientists compared recent observations with eggs from museum collections and determined that birds are laying their eggs 25 days earlier on average. The researchers then examined carbon dioxide records to estimate changes in temperatures in the region over the last century and found that the estimated increases in carbon dioxide, and hence temperature, correlated with the earlier egg-laying, suggesting climate change is the cause.

What do carbon dioxide levels tell us? There is a strong correlation between global climate change, temperature, and the concentration of carbon dioxide in the atmosphere. Observations show that when the carbon dioxide concentration goes up, the temperature rises too and the same in reverse, i.e. lower carbon dioxide concentrations mean lower temperatures are observed. Researchers can therefore use carbon dioxide concentrations to assess changes in the climate over time. The scientists in this study compiled atmospheric CO2 data from published data sets and used this information to estimate the levels for the years where data was unavailable.

OK, but how does that link to birds' nesting times? The researchers modeled the collected information on CO2 levels with data they complied on nesting times. They used data of the contemporary nests of 72 species found in the Chicago area and compared it to historical information on lay dates from museum records. This created two large sets spanning two periods: 1872 – 1963 and 1983 – 2015. Natural history collections and records are important resources for scientific research but potential difficulties can arise due to problems such as missing data points or sample bias.

Yeah, how can they make assumptions if some data was missing? Good point! This study found a novel way to address these concerns by using a statistical model, allowing to estimate uncertain quantities (it's called the Bayesian multilevel linear model). This helped the researchers account for inconsistencies in data collection and other variables. The team has found that 33% of the species studied had brought their laying date forward and that this was often correlated with changes in atmospheric carbon dioxide, suggesting climate change is to blame.

Emissions, birds and nesting. As the climate started warming birds started nesting earlier. Fidino, Nowak-Boyd, Strausberger, et al, 2022.
Emissions, birds and nesting. As the climate started warming birds started nesting earlier. Fidino, Nowak-Boyd, Strausberger, et al, 2022.

So what if birds nest a bit earlier? While birds laying their eggs a few weeks or days early might not seem like a big deal, it is only one piece of a much larger puzzle. Early nesting times will impact a bird's biological clock, which can alter migration as well as breeding. This could have a knock-on effect on the availability of food in the areas they spend the winter, where they stop during migrations, and also in breeding areas. Changes in some birds' nesting and laying behavior may also increase the competition with other animals for resources in a way that they didn’t have to before. These small changes across populations serve as a warning about climate change, which is likely to reduce the variation in life history among competitors, increasing competition not only in established communities but also in novel ones, impacting their survival and reproductive capacity.

OK, but does it affect humans? Well, yes. On a larger scale, lower survival and reduced reproduction in bird species will result in fewer birds acting as pollinators, seed spreaders, and general habitat maintainers which will have a devastating effect on ecosystems around the world.

Sorry, but climate research based on museum collections does not sound serious… I disagree! Collections provide valuable information but are currently underused. Future studies could make use of themto look into the past. This will not only enable researchers to observe already occurring changes but the information could be used to predict what will happen in the future under different climate change scenarios. The predictions then could be combined with other techniques such as stable isotope analysis of tissues and feathers. Why? To look at changes in the diet or geographical range of birds which in turn would help provide a larger picture on how climate change is affecting species as a whole as well as their interactions.


Climate change is not only causing birds to lay eggs early…

…. It is also causing birds to shrink in size while their wingspan is increasing. One interesting study examined 52 North American birds over nearly 40 years and found that as temperatures warm, birds' bodies are getting smaller.

And not just birds. Research has shown that climate change will cause a whole host of animals to shrink in size including mammals and fish. One study even showed that warming temperatures are causing several beetle species to become smaller over time.

But why are animals getting smaller and smaller? Well, evidence suggests that as the average temperature rises, smaller warm-blooded animals find it easier to keep cool. For some cold-blooded animals, warmer temperatures speed up their metabolism resulting in stunted growth.

Dr. Lindy Whitehouse has curated 10 papers, saving you 35 hours of reading time.


The Science Integrity Check of this 3-min Science Digest was performed by Chloe Todd.



Try Sparrow today

  • Read the latest science updates in just three minutes
  • Get five Digests emailed to you every week—100% free
  • No angle. No agenda. Just the facts.
  • Premium subscribers get access to the complete Sparrow library