In my first Signs of Summer this year (May 28, 2015) I mentioned a project that Deborah and I were participating in up at Harrison Hills Park. We had teamed up with a wonderful group of volunteers to monitor a set of thirty (or so) “bluebird boxes” that had been set up around the park six years previously by a Boy Scout troop. Our early data in May indicated that four boxes had bluebird nests and eggs (three of which already had nestlings) and five boxes had tree swallow nests (four of which had eggs but no swallow nestlings yet!).
I am happy to report that the work of the “Cavity Nesting Team” (so named because we were encouraging ALL native bird species that nest in enclosed spaces to utilize our nesting boxes) continued all summer, and that we had a very successful nesting season!
Some background information: Native cavity nesting bird species (eastern bluebirds, tree swallows, house wrens, Carolina wrens, titmice, chickadees, nuthatches, etc.) naturally use tree holes for their nesting sites. These holes are most often found in older, often dead trees and are typically abandoned cavities that have been chiseled out by woodpeckers. Any site management plan that favors woodpeckers (allowing dead trees to remain in the forest and not managing the forest or manipulating it into an even aged stand) will favor cavity nesting bird species.
Nest boxes are substitutes for these natural tree holes. Eastern bluebirds came under a great deal of stress in the past century. The influx of the alien invasive English sparrows and European starlings along with the habitat spread of the nest parasite, the brown headed cowbird, were major reasons for the bluebird’s numerical and distributional decline throughout the twentieth century. Human destruction of nesting and feeding habitats were also contributing factors to this decline. Human efforts to provide existing bluebird populations with suitable and secure nesting sites (“bluebird boxes”) have, however, been extremely successful in bringing this beautiful species back from the brink of extreme decline. . The North American Breeding Bird Survey reports that since 1966 eastern bluebird populations have increased by nearly two percent a year! The Cornell Laboratory of Ornithology estimates the worldwide population of eastern bluebirds (80% of which spend at least some time in the United States) at a very robust 22 million individuals.
The nest boxes at Harrison Hills were placed around the park in a very random manner. Easily accessed sites tended to have a greater number of boxes placed very close together compared to sites that were more difficult to get to. The lack of initial design in the placement of the boxes relative to the surrounding woods and to each other set up a very interesting experiment. Our plan was to carefully count the numbers of eggs and fledglings that the cavity nesting bird species utilizing our boxes produced and then correlate these nesting successes to the location and orientation of each box.
Bluebirds: We had 13 boxes that had bluebird nests and a total of 65 bluebird eggs. The number of fledges from these eggs was 48 (a 74% survival rate). Successful bluebird boxes were located about 30 to 40 feet from the edges of the woods (definitely not right on the edge of a field and definitely not in the center of the fields). Most bluebird-occupied boxes faced south (or southeast) with the exception of one box that faced north and two boxes that faced East. Bluebirds did not occupy boxes close to other bluebirds. Two bluebird boxes, though, were very close to (within 100 feet) occupied swallow boxes. Bluebirds reproduce in staggered waves (three reproductive time events (April/May, May/June, July/August). Reproduction in this species was not a rigidly timed event. Two boxes had cataclysmic failures (an adult bluebird found dead in one box, a black rat snake consuming eggs and nestlings found in another).
Tree swallows: We had 9 boxes that had tree swallow nests and a total of 31 tree swallow eggs. The number of fledges from these eggs was 22 (a 71% survival rate). Successful tree swallow boxes were further from the edges of the woods than the successful bluebird nests and were located more in the centers of the meadows. Two swallow nests were located near the park’s big pond. Two swallow nests were located within 100 feet of active bluebird nests. Tree swallows reproduce in a more organized (more rigid?) time frame. Almost all nesting occurred in a May/June time frame (only one nesting cycle for the season). Three boxes had both bluebird and tree swallow nests during the nesting season (two had April/May bluebirds and June tree swallow, one had June tree swallows and August bluebirds.
Nine of our boxes did not have nests through the summer. The southern edge of the northern meadow in the park had one third of our inactive nest boxes. These boxes were placed too close to each other and, possibly too close to the woods and too close to an actively used hiking trail. The vegetation around these boxes was also quite high. Two other inactive boxes were also in this northern meadow. They were located quite close to each other on the northern edge of the field. Five of the nine failed boxes were located in this northern meadow.
Several of these boxes also had abundant service berry bushes around them (one was actually encased in the branches of a service berry bush that had grown up under it!). Several of these boxes were also shaded by nearby trees in the afternoon. The branches of these service berry bushes are strong enough to support climbing nest predators. The densely intertwined branches of these bushes also conceal ground movements of possible predators. Recognition of these potential predator vulnerabilities may have kept bluebirds and swallows from using these nesting boxes.
Our plan for next year is to move the nine unused boxes into more optimal locations. For bluebirds, this will be 40 feet or so from the edge of the woods at least 200 or 300 feet away from other boxes. Optimally, they will be located in areas with very low (possibly even regularly mowed) vegetation. For tree swallows, we will put some boxes in the middles of the park’s meadows and down near the park’s ponds. We will avoid placing them in areas with dense, shrubby vegetation.
We moved the boxes last weekend! A really gratifying ending to our morning of pulling up the nest box poles and replacing any damaged boxes was the almost instant arrival of three male bluebirds to one of our freshly placed boxes! We literally had just walked away from the box when three bluebirds swooped in and proceeded to check out the top and the inside of the box (photo to the left)! A small percentage of bluebirds overwinter in our area, and we think that these three birds were checking out the new box for a possible winter residence! I will keep you posted on their activities!