Why Study Kestrels?

American kestrel © David Larson, Mass Audubon
American kestrel © David Larson, Mass Audubon

According to our Breeding Bird Atlas I and II, there has been a sharp decline in where American Kestrels breed, though the reasons are still being researched. However, a number of factors have become apparent as likely contributors to the problem.

Lack of Natural Cavities  

American Kestrels are secondary cavity nesters. This means that they nest in holes in trees, but they cannot excavate their own holes for this purpose. They must rely on other species such as Northern Flickers and Pileated Woodpeckers to create holes. Unfortunately, it’s becoming increasingly difficult for kestrels to locate holes in trees that are large enough to accommodate them and near enough to an open area for hunting.

Predation

Although American Kestrels are birds of open fields, they often live and hunt near wooded areas. Massachusetts is more forested today than it has been at any point in the last 200 years, which means that many forest-dwelling species are experiencing a huge upswing.

One such species, the Cooper's Hawk, specializes in preying on smaller birds including American Kestrels. Raccoons raid kestrel nests under cover of darkness, and squirrels love to take over cavities that kestrels could use for nesting. As more and more open space grows into forests, these forest-dwelling species put greater pressure on American Kestrels.

Contamination

Although DDT and many other harmful pesticides have been banned, a new chemical specter has arisen to haunt breeding raptors: flame retardants. Commercial flame retardants, which can be found in everything from textiles to household appliances, are becoming more prevalent in the environment.

According to a 2009 study by the Canadian Wildlife Service, captive American Kestrels experienced reduced fertility, thinner eggshells, and lower hatchling survival when exposed to the same levels of flame retardants found in wild birds¹.

Loss of Suitable Breeding Habitat 

Kestrels require wide open spaces to be able to hunt effectively. Pastures, hay meadows, old fields, and natural clearings caused by fire all offer excellent habitat for kestrels. Yet, these places are disappearing all over Massachusetts, either being swallowed up by development or growing back into forests.

As their preferred hunting grounds become smaller and more fragmented, kestrels can no longer find adequate territories where they can breed.

¹ Fernie, KJ, J Laird Shutt, RJ Letcher, IJ Ritchie, and DM Bird. 2009. Environmentally relevant concentrations of DE-71 and HBCD alter eggshell thickness and reproductive success of American kestrels. Environmental Science and Technology 43(6):2124-2130.