-Our results suggest that breeding productivity may decrease about 12% for the average bird species, the research reveals
Harare, Zimbabwe (CZ) – Rising global temperatures are making it harder for birds to know when it is spring and time to breed according to a new study published in Proceedings of the National Academy of Sciences.
A large collaboration, led by scientists at UCLA (University of California – Los Angeles) and Michigan State University has found that birds produce fewer young if they start breeding too early or late in the season.
With climate change resulting in earlier springlike weather, the researchers report, birds have been unable to keep pace.
And, the authors write, the mismatch between the start of spring and birds’ readiness to reproduce is likely to worsen as the world warms, which could have large-scale consequences that would be catastrophic for many bird populations.
Birds’ breeding seasons begin whenever the first green plants and flowers appear, which is happening earlier as the climate warms.
The authors stress that conservation strategies should address bird species’ responses to climate-driven shifts.
Determining if the earlier springs will pose problems for migratory birds has been a major goal of biologists for decades.
When it comes to raising their young, timing matters for birds. If they breed too early or too late, harsh weather could harm their eggs or newborns.
-But timing relative to food sources
If birds are looking for food before or after its natural availability, they might not have the resources to keep their young alive.
Using data from a large-scale collaborative bird banding program run by the Institute for Bird Populations, the researchers calculated the timing of breeding and the number of young produced for 41 migratory and resident bird species at 179 sites near forested areas throughout North America between 2001 and 2018.
Then, the authors used satellite imaging to determine when vegetation emerged around each site.
They found that each species had an optimal time to breed and that the number of young produced decreased when spring arrived very early, or when breeding occurred early or late relative to when plants emerged.
While the majority of birds were adversely affected by variations at the start of spring, several species countered the trend, demonstrating improved breeding productivity when spring began earlier.
Those species are mostly non-migratory species that can respond more quickly to the emergence of spring plants that signal the start of the breeding season.
By breeding earlier and without the time constraints imposed by migration, the study noted, non-migratory species may also be able to reproduce more than once per season.
But those species were the exceptions to the rule. Even most non-migratory species couldn’t keep up with earlier spring arrivals. Overall, for every four days earlier that leaves appeared on trees, species bred only about one day earlier.
For migratory species, that discrepancy means that the time between when they arrive at their breeding sites and breeding itself is likely to get shorter as springlike conditions begin earlier. Birds need time to establish territories and prepare physiologically for egg-laying and rearing their young so that change could cause even greater disturbances to reproduction.