Over the past two years I have written three blogs about our Cavity Nesting Team study up at Harrison Hills Park in northern Allegheny County. We followed up our very satisfying 2015 season with a second round of spring and summer-long monitoring of our 28 cavity nesting boxes, and we have just begun to organize and explore our 2016 data.
In 2015, thirteen of our boxes had bluebird nests with a total of 65 eggs and 48 fledges (a 74% survival rate). We also had nine boxes that had tree swallow nests with a total of 31 eggs and 22 fledges (a 71% survival rate). Nine of our nest boxes, though, did not have any nesting activity, and, so, using the placement of the utilized boxes as a guide, we relocated seven of these inactive boxes to try to make them more attractive to cavity nesting bird species. Our overall criteria for nest box relocation were quite straightforward: boxes too close together tended not have nests (so we spread out the clumped boxes) and boxes right on the edges of field (i.e. very close to surrounding woodlands) were not used (so we moved the boxes away from the extreme edges of our fields).
This year’s Cavity Nesting Team consisted of eight volunteers: Deborah and I and Sharon Svitek took turns monitoring the boxes in and around the “High Meadow.” Patrick and Mardelle Kopnicky checked the boxes around the “Bat House Meadow.” Chris Urik and Odessa Garlitz took turns monitoring the boxes at the park entrance and up in the field near the Environmental Learning Center, and Paul Dudek checked the boxes around the pond and soccer fields in the southern end of the park. Every box was checked each week, and then each observer uploaded their data to an on-line Google spreadsheet. Each week, Deborah compiled and distributed the growing data tables to each member of the team. Chris Urik also made GPS maps of the park showing the precise location of each nesting box.
As I have talked about before, 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, of course, are artificial substitutes for these natural tree holes.
So what did we see in our 2016 study?
First of all our nest box relocation project was very successful! The seven relocated, unused nest boxes from 2015 were all used by cavity nesting birds in 2016! Four of these boxes had bluebird nests, two had tree swallow nests, and one had a chickadee nest.
Bluebird nesting was more robust this year than last (in 2016, 17 boxes had bluebird nests, 83 eggs were counted and 63 fledges observed (a 76% success rate!)). Like last year, nesting activity in bluebirds had two seasonal peaks: one in late spring (May) and another in late summer (July).
Tree swallow nesting was less active in 2016 than it had been in 2015. Only seven boxes had tree swallow nests this year and only sixteen eggs were observed with just five confirmed fledges (a 31% success rate (compared to a 71% success rate in 2015)). It is possible that the hot, dry summer of 2016 reduced the food supply typically utilized by tree swallows to feed their young (primarily adult forms of insects that have aquatic larvae), or possibly the increased activity of house wrens in the park (described below) interfered with tree swallow nesting and may have even led to the active destruction of both tree swallow eggs and nestlings.
In 2016, three of our nest boxes had chickadee nests (in 2015 only one box had a chickadee nest). We observed a total of fifteen chickadee eggs and four confirmed fledges (a 27% success rate). All chickadee nesting was in May and early June. Two of this year’s chickadee nests were raided by nest predators (accounting for the eleven failed eggs).
In 2016, nine of our nesting boxes had house wren nests (no boxes from 2015 had house wren nests). Thirty-four eggs leading to twenty-three fledges were observed in these nests (a 68% success rate). Six of the nine house wren nesting boxes had been previously utilized (and nested in) by bluebirds (4 boxes) and chickadees (2 boxes).
House wrens and bluebirds have many similar nesting characteristics: they tend to nest in two seasonal cohorts (early spring (May) and late summer (July/August) and they tend to select nesting sites no more than 50 to 100 feet from a wooded edge. Possibly our box relocation project moved a significant number of our boxes into nesting sites also preferred by house wrens.
House wrens are one of the most common causes of nest failure in bluebirds, tree swallows and chickadees. They destroy eggs, kill nestlings, and even kill adult birds (typically then throwing the broken eggs or dead birds from the invaded nest so that they can start their own nesting cycle). Also, male house wrens attempt to attract females by building numerous “dummy nests” (piles of sticks sometimes stacked up on the active nests of other birds!). These dummy nests are an important sign of house wren activity and can be used to thwart house wren nest invasion. Since these dummy nests contain no eggs, they can be removed from the nest boxes without violating the Migratory Bird Treaty’s protections of native bird species. Prompt removal of these stick piles may keep a male house wren occupied in re-building the displays rather than getting down to actual reproduction.
Also, the literature on “wren guards” (various nest box modifications that are designed to repel house wren nest box invaders) stresses that it is the visual cues of the nesting birds entering and leaving a nest box that are the critical stimulations that then trigger the house wrens to attack the active nest. Possibly, turning all house wren utilized nest boxes to point their openings away from the surrounding woods (where the house wrens spend much of their time foraging for food and hiding in the covering vegetation) would make the visualization of the nesting birds entering and leaving the nest box less apparent and decrease the rate of house wren infestation.
So, we had a successful season! We fledged out lots of bluebirds but fewer tree swallows, but we have also, possibly, set up a potential plague of nest-preying house wrens! Next year, we’ll try to reduce the house wren activity and keep our bluebird numbers high! I will let you know how 2017 plays out!