Signs of Summer 3: The Bluebirds of Harrison Hills Park

Photo by D. Sillman

The Cavity Nesting Team is back for its third consecutive season in Harrison Hills Park in northern Allegheny County! The team (which includes Mardelle and Patrick Kopnicky, Sharon Svitek, Odessa Garlitz, Chris Urik, Paul Dudek, Kathy and Dave Brooke, and Deborah and I) monitor and maintain the 28 “bluebird boxes” scattered around the park and record weekly data on the nests, eggs, and hatchlings in each box. In 2015 and 2016 the team made some important observations on nest box location preferences for our cavity nesting species (including bluebirds, tree swallows, chickadees, and house wrens) and on seasonal timing of their reproduction and the influences of environmental variables on reproductive rates for each of our species. I summarized these data in my “Signs of Fall 2” (September 22, 2016).

The experimental objectives this year involve two ideas: 1. Can we orient the nest boxes in a way to reduce their use by house wrens? (House wrens are very active nest predators and nest box usurpers and have significant impacts on both bluebird and tree swallow reproduction). And 2. Are there any of our cavity nesting species utilizing natural tree cavities for their nests?

Photo by dfaulder, Wikimedia Commons

Our plan for Objective #1 involves active removal of house wren “dummy nests” from our nesting boxes. These dummy nests are built by male house wrens as displays designed to attract females. Since these nests do not have any eggs, we can legally remove them from the boxes and thus, hopefully, keep the males busy making their displays without getting down to actual reproduction. We also have oriented the openings of our field-placed nest boxes away from the surrounding tree/shrub ecotone. The house wrens inhabit these ecotones preferentially and are known to respond to the movement of other birds in and out of their nests by actively invading and preying on those nests. All of our boxes, then, have been turned to face the centers of their fields and meadows.

Our plan for Objective #2 involves close observation of the standing trees on the edges of our fields and meadows. Some of these trees may be of sufficient size and age to house cavity nesting birds. Many of these trees have also been worked on by pileated woodpeckers and have significant numbers of rectangular holes in their trunks. These observations will be on-going through the breeding season.

We are two months into our 2017 observations (time goes by so quickly!). What have we seen so far?

K. Thomas, Public Domain

We are coming to the end of the early spring bluebird reproduction cycle. This April/May burst of reproduction (which we also observed in 2015 and 2016) has involved bluebird’s nesting in eleven of our twenty-eight boxes with the production of forty-nine eggs and, so far, forty hatchlings (which are just now beginning to fledge). We have seen some nest predation (five hatchlings were killed in one of our boxes), but, overall, the rate of reproduction and success rate (percentage of eggs maturing into hatchlings and fledglings) is higher this year than in either of the previous years. We expect the bluebirds to take a reproductive pause as summer sets in (maybe because of food availability or maybe because of competition with other cavity nesting species?) but then carry out a late summer nesting cycle in July and August.

Photo by D. Sillman

Tree swallows have just begun their seasonal nesting. This timing also fits into our previous years’ observations. The swallows rely on aquatic insects to feed their nestlings and time their nesting to the June emergence of large numbers of these insects. The very dry conditions of June 2016 may have resulted in reduced insect numbers, and the swallows, which have been described in the literature as a resource-dependent nesting species, then, reduced their numbers of nests and numbers of eggs significantly from 2015 (which was a much wetter summer). We will watch with great interest how the reproductive rate of the tree swallows is influenced by this, so far anyway, wet spring and summer. We have thus far counted five swallow nests in our boxes.

The house wrens made nests in nine of our boxes in 2016 (there were no house wren nests in 2015). A total of thirty-four house wren eggs and twenty-three fledges were observed in 2016. The house wrens, like the bluebirds, nested and reproduced in an early spring and then a late summer timing cycle. So far, we have seen seven house wren nests and twenty-nine eggs! Four of the seven nests are in boxes located in the ”bat house meadow” in the northern section of the park. One wren nest was in a nesting box in a small meadow near the pond that had not had any birds’ nests at all throughout our three years of observation! Initially, it does not seem that our orientation of the nest boxes to point away from the surrounding trees and shrubs and toward the open spaces of the fields and meadows has been effective in reducing house wren activity.

Photo by A. Wolf, Wikimedia Commons

Last year we observed three chickadee nests in which fifteen eggs were deposited (there were, however, only four confirmed fledges from these eggs). The nesting timing of the chickadees in 2016 was in late May and early June. So far this year we have only seen one chickadee nest and that nest was usurped by a house wren. The next few weeks may be critical for successful 2017, chickadee nesting in Harrison Hills Park.

When I went out on my nest box circuit yesterday afternoon I also walked the edges of our three northern meadows (the bat house meadow, the high meadow and the purple martin field) where fifteen of our twenty-eight nesting boxes are located. I was looking for sufficiently large trees that may serve as natural, nest cavity locations for any of our cavity nesting bird species. I need to get out to these meadows in the morning to see if these is any evidence of cavity nesting activity. Right now all parental bluebirds, for example, are actively feeding their growing nestlings. If these are nests in the tree holes, I should see the comings and goings of the parental birds!

So, our 2017 season is going along well! The bluebirds are booming, and the tree swallows are following closely behind. The house wrens have made significant inroads into our nesting resources, though. Maybe we have to just take them along with the bluebirds and swallows!

More soon!

 

 

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One Response to Signs of Summer 3: The Bluebirds of Harrison Hills Park

  1. Paul Hess says:

    Very interesting summary, Bill. I’m looking forward to your future reports.

    Paul

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