The chicks will love these (Image: Greg7 / flickr)

Junk food is stunting the growth of young suburban crows, new research suggests. To make matters worse, and like some humans, crow parents opt to feed their young less nutritious food if it is easier to get.

The findings show how evolved behaviors can become detrimental as previously natural environments turn into suburbs.

For crows and other birds, suburban and urban environments provide rich, relatively predictable sources of food. Dumpsters, trashcans, or the unattended grocery bag all make for easier scavenging than the comparatively bare countryside. But while the leftover French fries and donuts a crow would find at these sources may be fine for adults, they can have a detrimental effect on growing young crows.


Poor diet

Rebecca Heiss and colleagues at Binghamton University, New York, found that not only were suburban nestlings smaller than their rural brethren, their levels of blood protein were also lower. The results suggest that their diets, while adequate in terms of calories, lack nutrients that nestlings need to grow to their full potential size.

“They didn’t look like they were malnourished, just smaller,” says Heiss, who along with her team spent months watching a population of nearly 40 nests. “This is a really well-watched population; we know these crows from egg to death,” Heiss explains.

When the nestlings were between 23 and 31 days old, Heiss and her team climbed up to the nests, dodging the dive-bombs of protective parents, to collect the baby birds. They took a quick blood sample from each, weighed and measured them, and then returned the nestlings. Back at the lab, tests revealed lower protein and plasma calcium levels in the suburban young compared with rural chicks.

Super snack

Heiss also offered a few of the nests a home-baked nutrient supplement: baby food, powdered eggs, and soaked cat food. The concoction had more calcium and protein than typical suburban fare; a fact that was reflected in increased growth in the suburban nestlings that ate it.

Surprisingly, however, the rural nestlings whose parents fed them supplement were smaller than those who ate natural diets, usually a combination of insects, seeds, and the occasional small animal. This suggests that crow parents will choose less nutritious food if it is more readily available.

“If someone throws out trash, then the crows eat the trash instead of, say, a baby rabbit,” says co-author Kevin McGowan.

‘Evolutionary mistake’

“Urban sources of food tend to be highly predictable,” says Reed Bowman of the Archbold Biological Station in Lake Placid, Florida.

According to Bowman, it makes sense that adult birds would opt for easily available, less nutritious, food for themselves, but he finds it puzzling that they would offer the junk food to their offspring. “If bad food makes nestlings worse off, why do parents do it?” he asks.

Bowman thinks that it’s an “evolutionary mistake,” that crows evolved to be opportunistic feeders, which helps them in natural environments, but may be detrimental as humans continue to urbanise the rural landscape.

It’s not known if the size differences observed in young crows will persist into adulthood. It is also possible that smaller young crows may not be able to compete with more robust challengers for territory, preventing them from surviving to adulthood. More research is needed to find out how the nutrient inequalities play out over the entire life-span of the crows, say the researchers.

Journal reference: Ecological Applications (DOI: 10.1890/08-0140.1)