A number of readers have contacted me recently with a concern about bird feeding. Holiday travel means that our bird feeders may be depleted while we are away, depriving birds of our handouts. Readers are asking if we are doing harm to birds by providing and then removing food.
This query requires us to answer two questions. First, do birds benefit from bird feeding? Second, do birds become dependent on bird feeders?
The answer to this first question is yes. One line of evidence comes from a technique called ptilochronology, a daunting word that refers to the rate of feather growth. Did you know that you can see daily growth bars on a feather? Each contour feather a bird produces has a record of how quickly it was formed.
Tom Grubb and his students used this technique to examine the impact of bird feeding. They captured Downy Woodpeckers, White-breasted Nuthatches, Carolina Chickadees and Tufted Titmice in the winter in Ohio. The researchers plucked one of the tail feathers of each bird. The birds quickly started replacing that missing feather.
Half of the birds had access to bird feeders and the remainder were found in areas where only natural food was available. After a month or so, the birds were recaptured and their regrown tail feather was examined. The birds with access to supplemental food regrew their tail feathers at a faster rate; the daily growth bars were longer in these well-fed birds. So, bird feeding increases the nutritional status of birds.
Margaret Brittingham and Stan Temple examined the impact of bird feeding on the winter survivorship of Black-capped Chickadees in Wisconsin. They banded over 500 chickadees. Some populations were given access to unlimited sunflower seeds and some populations had to depend on natural food. Over the course of three winters, Brittingham and Temple found that winter survivorship of banded birds with access to supplemental food was 67% compared to 37% for birds without sunflower seed handouts. That’s a pretty striking result.
The researchers found that the greatest risk to the control (unfed) birds was in the coldest months with more than five days with subzero temperatures. Chickadees with supplemental food were also heavier than the control birds.
These results have been corroborated in similar studies done in Pennsylvania and Ontario.
Birds therefore benefit from the food we provide for them. But is there a risk that birds become dependent on our handouts? The answer to that question is no to the best of our knowledge.
We return to research done by Brittingham and Temple in Wisconsin. Having shown that bird feeding increases winter survival of Black-capped Chickadees, these ornithologists set up an experiment to test for feeder-dependence. In one area, chickadees had been given sunflower seeds continuously for several years. In the second area, no bird feeders were ever present. The authors took away the bird feeders from the first area where birds had been feeding on sunflower seeds for years and monitored winter survivorship of both populations.
If the population given sunflower seeds in previous years had a lower survivorship than the population with no bird feeders, one could claim that the birds in the first area had become dependent on the sunflower seeds. But there was no difference in survivorship for the two populations. The previously fed chickadees did as well in the following year feeding on natural food, as the unfed chickadees did that never experienced the bounty of bird feeders.
Similar studies have not been done for other North American species that frequent feeders but I expect that results would be similar. Depending on a single source of food is risky for any winter bird. Winter songbirds commonly range over areas of 10 to 25 acres. Much of this area is regularly patrolled and food is taken from a number of different parts of their winter area.
[Originally published on November 30, 2015]