May 2017

Cavity nesting birds

Marla Mertz, Marion County Conservation

Some of our most beloved birds that return to Iowa as their breeding grounds have been experiencing challenges in their habitats. There are numerous species of birds that rely on tree cavities and snags in our woodlands and river corridors. Some species have been assisted by human built housing and others have recovered from disastrous declines. What is the real reason for concern? The beginning of decline for many species begins with the loss of habitat. Weather, predator/prey relationships, and other mortality factors may impact nesting success in a given year, but, normally do not have long-term effects on populations.

Wood ducks, purple martins, and bluebirds are some of the most talked about birds that have required human assistance to help them recover from drastic decline. Without the help of man-made nesting structures and legal protection, these birds would not be so viewable today and playing their role in our environment. Other notable species that have received attention lately include the peregrine falcon and North America’s smallest falcon, the American kestrel.

So, just what is a “cavity nester?” They are North American birds that will excavate nesting holes or use holes that have been created by other species in deteriorating or dead trees. Some refer to woodpeckers and flickers as primary cavity nesters as they drill and hollow out nesting and roosting spaces. The secondary cavity nesters are the birds (and some mammals) that utilize the holes created by the primary nesters or have been formed by decay, insects, or breakage. Examples would be bluebirds, kestrels, screech owls, barred owls, and wood ducks.

Decaying trees are often called “snags.” The removal of deteriorating trees and clearing of woodland areas has been mainly a practice to make an area aesthetically pleasing, removal due to forest insect pests, or areas that could result in safety hazards. Other environmental pressures and management practices harvest large dead or dying trees that result in adverse effects within the ecosystem and effect some of our important species like nuthatches, woodpeckers, flycatchers, swallows, and owls.

What’s all the fuss? The majority of cavity-nesting birds are insectivorous (eat primarily insects). In the spring, summer, and fall in Iowa, they make up the largest part of the forest-dwelling bird population. Birds like woodpeckers are incredibly important predators of many species of bark beetles and other insect species that may be tree-killing. Other insects and devastating caterpillar pests are the major diets of other birds like the cuckoo.

The following summary is from “Cavity-Nesting Birds of North American Forests” Forest Service/U.S. Department of Agriculture. (Massey and Wygant 1973). Bruns (1960) summarized the role of birds in the forests:

“Within the community of all animals and plants of the forest, birds form an important factor. The birds generally are not able to break down an insect plague, but their function lies in preventing insect plagues. It is our duty to preserve birds for esthetic as well as economic reasons... where nesting chances are diminished by forestry work.... It is our duty to further these biological forces [birds, bats, etc.] and to conserve or create a rich arid diverse community. By such a prophylactic... the forests will be better protected than by any other means.”

Since my tenure (almost 35 years) of working within a natural resource environment, there have been mentors, biologists, and technicians concerned about many cavity nesting species. Their efforts that have influenced me include improving habitat, nest boxes, bird counts, banding and educating the public through programming, and written articles about the importance and concerns of cavity nesters. Some of those species included in Marion County and Iowa were, barn owls, kestrels, wood ducks, bluebirds, purple martins, and peregrine falcons. It wasn’t until recently, I was noting a resurgence of kestrel nest boxes being placed along major Interstates...meaning the kestrel nest box studies were continuing and expanding. I never kept current with the research that was being compiled through the breeding bird atlas, their populations in Iowa were not of grave concern as some of the other species that needed assistance.

Here are a few facts and concerns of the American kestrel, also known as the sparrow hawk. The American kestrel is North America’s smallest falcon, an aerial artist, and a connoisseur of small rodents, sparrows, and insects. They are incredibly adaptable to human activity and habitat modifications, and can often be found in towns and cities. These birds can see ultraviolet light and are often seen along power lines and hovering along roadways and medians searching for urine trails of voles, one of their main food sources. Old trees, church steeples, barns, rooftops, parks, and grasslands are common nesting and hunting sites. Nest boxes have been a welcomed gesture for the kestrels, especially around the areas that have enough hunting areas available to support their hatchlings. Sometimes there will be four to seven hungry mouths to feed. As the time nears for the young to fledge, they have already donned their adult plumage. It is hard to determine which are the adults and which are the young, unless you observe the begging and calling of the young in training.

Nationally, the American kestrel is a species of concern, even though local reports say they are of least concern. Years of observation, research, and the “villages” to assist with citizen science projects can unfold answers as to why there is a decrease of the kestrel’s population. Read 'Open Mice: Kestrels - An Iowa Legacy' for a young birder's perspective. Some thoughts behind the American kestrel decline are European starlings taking over nest boxes, pesticide concerns, other raptors, i.e. owls and accipiters, stress, and nest abandonment. Only time can tell the real reasons and help shape a conservation plan.

Learn more about American kestrels, nest box plans, and kestrel studies in the following links:

You can help too!

Share the following link with individuals and organizations that are properly permitted to collect feathers from migratory birds!

Nest box plans

Kestrels along with screech, barn, and barred owls are cavity nesters and will accept a box in place of a natural tree cavity. Screech owls and kestrels will do well if there are no great horned owls nesting in the same area... great horned owls will predate on screeches and kestrels.

Barn owls need good grassland / CRP habitat to nest this far north (central Iowa). Barn owls will nest in available buildings, but readily adapt to nest boxes.

Barred owl's nesting territory may overlap a great horned owl's territory if the habitat supports a good supply of various sizes of prey. Great horned owls are bigger and if there is larger prey -- bunnies and squirrels -- that leaves the mice and voles for the barred owls.

Several plans available from the Natural Resources Conservation Service

In Iowa, your local county conservation board may have nest box plans and more information.