May 11, 2015
Thanks to Emma Jesch, guest student blogger, for this excellent post about how feeder birds use shade to avoid predators. Providing a little cover around you feeding area could be a big help to your feathered visitors.
Winter sparrows: sun or shade?
A cluster of sparrows, sable and striped feathers puffed up to conserve heat, peck beneath a shady feeder, searching for leftover sunflower seeds. The frigid air sends a shiver through the flock, but they stick to the cool shade, away from the dappled sunshine. Why would these birds give up the opportunity to sun themselves and dull the bite of the cold? According to researchers from Indiana State University, although foraging for nuts and seeds in direct sunlight is thermally advantageous, the birds become much more vulnerable to hungry predators.
During blustery winters, the high winds and low temperatures lead to a reduction in the birds’ metabolic rates. Consequently, they may need to spend more time pecking around for food to refuel. But the sparrows have a tool to their advantage: the sun. The heat from solar radiation can boost metabolic rates and reduce the amount of energy loss from foraging in the winter. But the sun is also good news for predators, as their prey may have a harder time seeing them with the sun in their eyes, making them more vulnerable to attack.
Dr. Jennie Carr and Dr. Steven Lima from Indiana State University conducted a study to see how this trade-off between preserving energy and increasing vulnerability worked in flocks of wintering sparrows.
The researchers spread a thin, even layer of finely ground cornmeal on a concrete pad surrounded by boards they could manipulate to shade different areas of the pad. They then filmed the site for one hour each day when the sun was at its highest point in the sky.
Drs. Carr and Lima found that the birds, which included dark-eyes juncos, American tree sparrows, song sparrows, and cardinals, preferred to forage in the shade, regardless of where on the pad the shade was located. This implies that safety from predators is more desirable than sunny patches, the researchers said.
They also found that the birds tended to face south, towards the sun, regardless of sunny, shady, or cloudy conditions. This result may be a response to the behavioral tactics of predators, which often attack “out of the sun” to make it harder to be seen. The split second advantage to facing the predator directly could make the difference between life and death.
What does this mean for feeder owners? Make sure to provide a bit of shady cover in your feeding area so the small birds have a better chance of seeing predators before they strike.
Reference: Carr, J. M. & Lima, S.L. (2014). Wintering birds avoid warm sunshine: predation and the costs of foraging in sunlight. Oecologia, 174, 713-721. doi: 10.1007/s00442-013-2804-7