Signs of Fall 3: Cavity Nesting Team, Year Four Completed

Photo by D. Sillman

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The Cavity Nesting Team has completed its 2018 observation season up at Harrison Hills Park in northern Allegheny County. This year we had twenty-nine nesting boxes scattered across the park. Our 2018 monitoring began at the end of March and ended in late August. Data from our three previous study years have helped us to learn how to optimally locate nesting boxes for bluebirds, and this year’s record number of nesting boxes (19) utilized by bluebirds shows that our criteria for box placements were ecologically sound.

The ten volunteers who make up the Cavity Nesting Team (see Signs of Summer 6, July 12, 2018) checked every box in the park at least once a week and recorded the presence of  and types of nests, numbers and types of eggs, numbers of nestlings, and numbers of fledglings . A total of 540 observations were recorded in our on-line spread sheet.

We saw our first bluebird nest on April 19 in nesting Box Q near the park’s Environmental Learning Center, and the first eggs were seen in that nest a week and a half later (April 28). Two weeks later (May 14) these eggs had hatched into nestlings and by then a large number of other boxes throughout the park were being utilized by nesting bluebirds. Our last nest was also built in Box Q (on July24) and four eggs were observed on July 29. These eggs all hatched and developed into young bluebirds that fledged on August 25.

The bluebirds at Harrison Hills nest and reproduce in two distinct time intervals (one in early summer (May/June) and the other in late summer (July/August)). In our previous years’ studies the early summer reproducing cohort produces two thirds of the eggs and two thirds of the fledglings for the season, and this year was no exception (69% of all eggs were laid and 63% of the bluebird fledglings were produced in the early summer of 2018).

K. Thomas, Public Domain

We observed a record number of bluebird nests in 2018 (23 nests), but also had a record number of bluebird nests that had no eggs (six) and ones that had a high degree (50% or more) of egg mortality (also six). The impacts of the eggless nests and large number of lost or undeveloped eggs reduced the overall egg production for the season to the second lowest total of the past four years (only 67 eggs) and reduced the total number of bluebird fledges to the lowest number we have observed over the four years of our study (only 46). Also, the early summer “success rate” for our the bluebird eggs (percentage of eggs that fully developed into fledges) was the lowest we have observed in our study (only 63%).

Why did we see an apparent breakdown in the nesting and fledging success of our bluebirds? The weather this summer was very different from any of the previous years. We had record high temperatures in May (average daily temperature in May was seven degree F above historical averages) and extremely high rainfall in the months of April (148% over historical average), June (193% over historical average) and August (152% over historical average). We are not sure if these unusual weather conditions contributed to increased activity of nest parasites or predators or if they might have directly stressed either the adult birds or the nestlings.

Our 2018 bluebird observations, then, had negative and also some positive components. The reduced egg and fledgling numbers and lowered egg to fledgling “success percentages” were disturbing. The Team will continue to look over the data to try to come up with some hypotheses to explain this observations. Our large number of boxes that had bluebird nests, though, and the park-wide distribution of these bluebird-utilized boxes indicate the ecological soundness of box locations and the quality of the entire park as a habitat for nesting bluebirds.

K. Thomas, Public Domain

Four nesting boxes had tree swallow nests and 20 eggs were observed. All of these nesting boxes were located in the north end of the park. None of these swallow-utilized boxes, though, were located near the large pond in the south end of the park. In 2015, three nesting boxes near the pond had had tree swallow nests, and we considered these boxes to be optimal for swallows because of the proximity of abundant flying insects (dragonflies etc.) over and around the pond. None of these pond-area nesting boxes, though, have subsequently been used by tree swallows! We are still uncertain why the swallows are avoiding these seemingly optimal nesting sites!

The tree swallow eggs were laid in the expected single, “mid-summer” time period that followed the initial blue bird reproductive cycle but preceded the bluebirds’ later “late summer” second reproductive event. This year’s tree swallow egg total was very comparable to our 2015 observations and was a significant rebound from the very low tree swallow eggs production of 2016 and 2017. We have speculated that the dry summers of 2016 and 2017 inhibited the emergence of the aquatic insects upon which the swallows rely to feed their nestlings. Tree swallows are known to be resource-dependent reproducers. The very wet summer of 2018 must have generated an abundant food base for the swallows, and they responded with a solid reproductive effort. Further, all twenty tree swallow eggs resulted in viable nestlings, and all twenty nestlings successfully fledged. This was the first 100% success percentage that we have recorded in our nesting box studies!

Photo by dfaulder, Wikimedia Commons

House wrens are a native bird species and a common cavity nester. They are also, though, very aggressive and destructive of the eggs, nestlings and fledglings of many other cavity nesting birds (including bluebirds). For that reason, we set up some protocols this year to try to discourage house wren reproduction. In past years we have observed some bluebird and tree swallow egg and nestling destruction by invading house wrens.  This year, one of the wren nests displaced a freshly built chickadee nest. The wrens usurped the nest from the chickadees and laid their eggs in the formed nest (it is not known if the chickadees had laid eggs or if chickadee nestlings had hatched in the nest, but if they had, they were destroyed by the wrens).

Nine nesting boxes broadly distributed across the park had house wren nests and thirteen eggs were observed. Of these thirteen eggs, though, only four developed into fledglings (a 31% success percentage). In past years, the house wrens nested and reproduced concurrent with the early bluebird nesting cycle (“early summer”).This year’s wren reproduction, though, occurred later in the season (corresponding more to the middle summer reproduction cycle of the tree swallows). Possibly this delay was due to both the Team’s strategic interference with the wrens’ “dummy” display nests (in an attempt to discourage wren reproduction) and also to our moving a number of nesting boxes away from the park maintenance area (an hypothesized habitat refuge for the wrens).

We are satisfied with our house wren experiment. Distracting the male wrens and forcing them to continuously rebuild their display nests may have been sufficient to reduce their potentially explosive reproductive potential.

House sparrow nestlings. Photo by P. Kopnicky

Also in 2018, for the first time in our cavity nesting box study, we have observed house sparrows (“English sparrows”) in one of our nesting boxes. These alien, invasive birds are very destructive (they invade established bluebird and tree swallow nests and destroy eggs and kill both nestlings and adults). The location of the park well removed from human habitations has insulated us from these birds, but here in the fourth year of our study they have finally shown up. Four eggs developed into four fledglings in one of the boxes near the Environmental Learning Center. We think that the house sparrow nest was built on a bluebird nest and possibly even on top of the dead bodies of some bluebird nestlings or adults. Since house sparrows are not native species, we were well in our legal rights to dispose of their eggs and even their nestlings, but none of us had the heart to do so. We hope that the fledged house sparrows will fly off to a habitat more suited to their needs (there is a McDonald’s and a Burger King not all that far away in Natrona Heights). We will, though, keep an eye out for them here in the park!

Stay tuned for Cavity Nesting Team 2019! We will have more ideas to test and more beautiful birds to encourage and count!