Over the past three years I have written a number of blogs about the work of our Cavity Nesting Team in Harrison Hills Park in northern Allegheny County. The Team has twenty-eight nesting boxes scattered across the park that are monitored weekly from the first of April through the end of August. The Team’s goal is to encourage the reproduction of bluebirds and other cavity-nesting species and to better understand the dynamics of their biology and ecology. Data from our 2015 and 2016 studies helped us design two experiments for 2017
This year’s Cavity Nesting Team consisted of ten volunteers: Sharon Svitek, Deborah and I took turns monitoring the boxes in and around the “High Meadow.” Patrick and Mardelle Kopnicky and Dave and Kathy Brooke 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. After each box was checked the observers 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.
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 or dying, trees, and they are typically abandoned cavities that have been chiseled out by woodpeckers. Any site management plan that favors woodpeckers (like 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 2017 study?
Bluebird reproduction was very comparable to 2016. Half of our 28 boxes had bluebird nests (and 10 of these boxes also had bluebird nests in previous years). There were slightly fewer eggs this year (81 this year versus 83 in 2016). Overall fledgling numbers, though, and fledgling success was higher this year than last (70 fledglings in 2017 vs. 63 in 2016 (86% success vs. 76%).
There has been a steady decline in the number of nest boxes used by tree swallows in our three annual collections. This year only 3 boxes had confirmed tree swallow nests (compared to 9 boxes in 2015 and 6 boxes in 2016). Further, only two of these nests actually had eggs. There were no observed tree swallow nestlings/fledglings in 2017. This was due to difficult viewing conditions in these two nests (they were deep nests with many lining feathers, and the brooding adults refused to leave nest during observation) rather than to there not being any nestlings or fledglings to observe. But no data is available on the “egg outcomes” for these two nests. It is reasonable, though, that these 9 eggs fledged far fewer than the 22 tree swallow fledges of 2015!
We hypothesized that last year’s decline in tree swallows may have been due to a hot, dry summer that reduced the mid-summer supply of insects (especially insects that have aquatic larvae) upon which the parental tree swallows rely to feed their young. The weather of 2017, though, was normally wet and moderate in temperature. Aquatic insects should have been quite abundant for the June nesting tree swallows.
We also discussed the possible impact of purple martins on the tree swallows (purple martins returned to Harrison Hills in 2015 after a long absence). There are no reports in the scientific literature that describe any negative interactions between purple martins and tree swallows. The director of the Harrison Hills purple martin project (Ken Kostka) indicates that martins take larger insect prey at higher altitudes than tree swallows, so competition for food resources does not seem a reasonable explanation for the apparently declining numbers of tree swallows.Some nesting pairs of tree swallows, though, have been observed in the purple martin communal nest complex near the Environmental Learning Center. Exactly how many tree swallows have nested with the martins has yet to be determined. Could the tree swallows of Harrison Hills Park be using these communal, martin nests instead of the isolated nesting boxes and thus be avoiding our weekly egg/fledgling counts?
The increased presence of house wrens may also explain some of the decline in tree swallow numbers. Eight nesting boxes this year had house wren nests (compared to 9 in 2016). The only year we observed robust tree swallow nesting and reproduction (2015) we had no house wrens nesting in our boxes. The house wrens concentrate their nesting just prior to the onset of the tree swallows’ reproduction. Possibly their presence kept the tree swallows out of our nesting box system.
House wrens are extremely aggressive and destructive birds. They 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 then 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!).Our experiments this year involved devising ways to reduce the impact of house wrens on our other cavity nesting bird species.
Experiment #1: we tried to orient the nest box openings away from surrounding vegetation and edge ecotones (habitats in which we hypothesized house wrens were likely to lurk). Reducing the visual stimulation of nesting birds entering and leaving their nest box, we hypothesized, might reduce the frequency of nest parasitism. This re-orientation of the box openings, though did not have any effect on house wren activity.
Experiment #2: we planned to remove the house wren “dummy nests” to discourage house wren reproduction. This also was not successful. Timely dummy nest removal would have required more frequent interventions with our nest boxes than our sampling scheme allowed. Our boxes were significantly and comparably (compared to 2016) utilized by house wrens for reproduction possibly (as I indicated above) to the detriment of the tree swallows.
So, our nest boxes are serving our bluebird population very well. We are trying to figure out why tree swallow numbers are so low, and would like to come up with an effective plan to reduce the use of our boxes by house wrens. All that will part of 2018!
Happy Fall, everyone!