Experiences in the first days and weeks of life can have a profound impact on humans — and birds. A new study led by Sage Madden, a graduate student in evolution and ecology at the University of California, Davis, shows how weather conditions and family dynamics affect the growth of barn swallow (Hirundo rustica erythrogaster) chicks.
By tracking over 100 barn swallow chicks during their first two weeks of life, Madden and working with Professor Gail Patricelli showed that the chicks' growth was stunted by cold spells if experienced within five days of hatching, if they were the smallest sibling, and if their parents brought less food back to the nest. Hot weather also stunted the chicks' growth, during both early and late development, and parents were unable to buffer their chicks against the heat by feeding them more.
The study, published April 22 in PLOS One , provides insight into the effects of climate change on chick development, which could influence their future survival and reproduction.
"To understand how extreme temperatures affect animals, we can't just look at populations as a whole. We need to look at what's happening to individuals and what makes some more resilient or vulnerable than others," said Madden, who is a fifth year Ph.D. candidate in the Graduate Group in Ecology .
Credit: Sage Madden
A delayed start to the internal thermostat
Like mammals, birds can regulate their body temperatures, but not until a few days after they hatch. In their first few days of life, chicks are as vulnerable to environmental temperatures as 'cold-blooded' animals like reptiles.
"Birds are both ectotherms, or 'cold-blooded', and endotherms, or 'warm-blooded', in the same lifetime," said Madden. "It's not only that they're developing insulation via their feathers; they also develop physiological mechanisms to cool or heat themselves about a week after they hatch, like shivering and panting."
Climate change might be making those first few days harder for chicks. Previous studies have shown that temperature extremes affect nestling growth differently across climate zones, but little is known about the impacts of local temperature fluctuations on chicks within the same population or nest.
"Even at these small scales, some individuals are doing better than others, and we don't really know why," said Madden. "We wanted to zoom in and ask whether there are certain stages of a chick's development when they're most vulnerable, and whether a chick's social environment might affect how they respond to temperature."
A risky lifestyle
Though temperature extremes likely affect all bird species, barn swallows' lifestyles might make them more sensitive to the weather — and climate change — which makes them a good species in which to study these questions, Madden said.
Barn swallows are aerial insectivores, meaning they catch and eat insects while flying. Because insects tend to hide when it's too hot, cold, windy or wet, insect-eating birds have a harder time finding food during inclement weather.
Barn swallows lay their eggs in cup-shaped mud nests that are open to the air, exposing their chicks more to environmental conditions than birds that nest in shielded tree cavities. And like other songbirds, newly hatched barn swallows are featherless and confined to their nests, unlike bird species such as ducks and geese, which hatch already feathered and mobile.
Tracking chick development
The team monitored the development of 113 barn swallow nestlings from 31 nests in Boulder County, Colo., from before hatching until the chicks were around 13 days old.
To continuously monitor the local temperature, the researchers placed thermometers within a foot of each nest. They also investigated whether family dynamics — how big a chick is relative to its siblings and how often parents bring food back to the nest — exacerbates or alleviates temperature stress.
Their results revealed that barn swallow chicks are sensitive to extreme lows, highs and fluctuations in temperature. Lower minimum temperatures were associated with stunted growth, but only if chicks experienced these cold spells during early development (within five days of hatching). This chilling effect was exacerbated for chicks whose parents provided less food and for the littlest chick in each nest.
"A lot of birds, including barn swallows, have a pattern where one nestling will hatch a day later than all the others, so there's one chick who is smaller than its siblings, and they are at a disadvantage when it comes to things like getting food," said Madden. "This disadvantage seems to also impact how vulnerable they are to temperature."
In contrast, all the nestlings were adversely affected by both high maximum temperatures and temperature variability, regardless of their age during the heat wave or how often their parents fed them.
"We found that there's this differential vulnerability to cold, but everybody is vulnerable to heat, though the mechanisms behind heat stress probably vary during different stages of development," said Madden. "Very young chicks might overheat because they can't regulate their own body temperature, whereas older chicks can regulate their temperature, but cooling themselves down requires them to pant, through which they lose a lot of water and can become dehydrated."
These results are a step toward understanding how the impact of temperature on nestlings is shaped by timing and social conditions.
"Our study provides a picture of how different aspects of the thermal environment affect nestling growth, and how social factors may buffer vulnerable individuals from human-induced environmental change," Madden said.
Now, Madden is studying how another aerial insectivore that builds cup-shaped mud nests—the black phoebe (Sayornis nigricans) — copes with extreme weather and urbanization.
"This project actually inspired my entire dissertation," said Madden. "It got me really interested in how urbanization and temperature combine to affect birds, and how birds are dealing with that in real-world settings."
Additional authors on the study are Rebecca Safran, Sara Garcia, and Zachary Laubach, University of Colorado Boulder.
The work was supported in part by grants from the National Science Foundation.
