Dangerous adaptation to global warming: If insectivorous birds breed earlier, they risk starving their chicks, according to a long-term study on a species of swallow. The frequent cold spells in early spring are therefore problematic: in bad weather there are hardly any flying insects around and there is therefore too little food for the brood. In addition to the general decline in insect populations, these daily effects could also be behind the particularly strong population collapses in the insectivorous bird species in Europe and North America, say the researchers.
Climate change gives us unusually warm days earlier in the year. Many studies already show how nature reacts by bringing its development forward. You’d think that everything just starts a little earlier. But this shift is problematic, as studies have already shown. They have shown that some biological systems that rely on optimal timing get out of step. For example, the early start of spring can lead to asynchronous developments in interdependent species. For example, some animals wake up from their hibernation when there is hardly any food, or plant species bloom when their pollinators are not yet on the move.
Problematic freak weather
Another problem is the moodiness of early spring: there is a decoupling between the general warming of spring temperatures and the occurrence of cold snaps. The unusually warm spring days often alternate with very cold conditions. This is problematic for many organisms. For example, a bloom triggered particularly early by heat peaks can be destroyed by subsequent cold spells. As the research team has now found, these freak weather conditions of early spring can apparently also impair the breeding success of some bird species.
The focus of their study was swallowtail swallows (Tachycineta bicolor) in the US state of New York. As with the European representatives of the swallows, they are migratory birds that breed in the north and winter in southern climes. Their breeding success depends on the availability of flying insects, with which they feed their chicks in spring. As part of the study, the researchers examined data from over 30 years on the availability of food and the breeding behavior of the swallows and linked them with weather data. “Long-term studies like ours are essential to understand how and why types of climate change are affected,” says co-author Maren Vitousek of Cornell University in Ithaca.
Hunger at cold spells
As the researchers report, in the course of global warming the swamp swallows have brought their average start of breeding forward by three days every ten years. In this context, the analyzes of the weather data showed that the chicks from the clutched clutch are much more exposed to adverse weather events than they were 30 years ago. The researchers were able to document that there is a sharp drop in the number of flying insects on corresponding days with low ambient temperatures.
This in turn has clearly negative effects on the breeding success: Even a single bad weather event can reduce the chicks’ chances of survival by 50 percent, according to the team’s results. “The decoupling between the occurrence of cold snaps and the general warming of spring temperatures can significantly impair reproductive success and threaten the long-term survival of populations,” summarize the researchers. “Our results suggest that animals that depend on food, the frequency of which can change rapidly due to the weather, are particularly at risk from climate change,” says Shipley.
The study could also provide clues as to why the populations of flying insectivorous birds such as swallows, swifts, flycatchers and goat milkers are declining faster than other groups in large parts of North America and Europe. Apparently these bird species are hit by a double blow: “The widespread decline in insect populations hits these species particularly hard. But apparently there is also a factor that is not connected with the general change in the abundance of insects: The change in availability over a short period of time also causes problems for the birds, “says Shipley.
Source: Max Planck Institute for Animal Behavior, specialist article: PNAS: 10.1073 / pnas.2009864117