Photo by D. Sillman
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Over the past six years I have written a number of articles about our Cavity Nesting Team study up at Harrison Hills Park in northern Allegheny County. Each year of our study has contributed significantly to our understanding of the ecology and reproductive biology of two especially important cavity hole nesters: bluebirds and tree swallows.
Our findings from 2015 helped us better understand the optimal location variables for our nesting boxes (and our relocated 2016 boxes were almost all utilized for nests!). Our 2016 data helped us design two experiments for 2017 in which we tried to regulate house wren nesting in our boxes (house wrens are very destructive to nesting bluebirds and tree swallows). The results of our experiments in 2017 were not very successful, but we re-tuned some of our 2017 ideas for the 2018 season and achieved a sustainable equilibrium between bluebirds, tree swallows and house wrens.
In 2018 we had a very warm and wet spring and summer, and, although we had a record number-to-date of bluebird nests (23), we also had a record number of nests with no eggs (6) and ones that had a high degree (50% or more) of egg mortality (also 6). Consequently, we had the second lowest number of eggs in any year of our study (just 67) and the lowest number of fledglings (only 46). We are not sure if these unusual weather conditions contributed to increased activity of nest parasites or predators or if the heat and the rain might have limited the food supply or directly stressed either the adult birds or the nestlings.
Seventeen year cicada. Photo by M. O’Donnell Flickr
In 2019 we experienced the emergence year of our local seventeen year cicadas! We weren’t sure if the June explosion of cicadas would have a positive or a negative effect on the cavity nesting species. Harrison Hills Park, though, had a great abundance of seventeen year cicadas! We were very curious if the bluebirds or the swallows would eat the cicadas or if these large, flying insects would interfere with the birds’ food gathering or nesting patterns. We also planned through our 2019 collection season to be very diligent in removing the display nests (also called “dummy nests”) that the male house wren builds to convince a female that he is worthy of reproduction in an attempt to discourage their mating.
Bluebirds and tree swallows have a very specific mating and nesting pattern that has been followed each of the years of our study. Bluebirds nest early in the spring (April and May) and produce about two thirds of their yearly eggs and fledglings in this reproductive event. They then nest again later in the summer (July and August) producing the rest of their annual eggs and fledglings. Tree swallows mate and nest in between these two bluebird nesting phases (in late May and June)! This pattern reduces competition for nesting sites (many of the early bluebird nesting boxes are then used by the swallows and a few are subsequently used by the late nesting bluebirds!
In 2019, this cycle was compressed into a much earlier time frame. The bluebirds started nesting in March and then re-nested in June, and by mid-July all of the bluebird reproductive activity was over for the year! The swallows, still fitting their nesting in between the bluebird nesting phases, started nesting in late April and early May and were done by the second week in June! We had a record number of bluebird nests and eggs (24 nests and 101 eggs), and 7 tree swallow nests with 32 eggs. Once again, though, the tree swallows mainly nested in the northern parts of the park away from the large pond in the south! The pond would seem to be an ideal habitat for the swallows, so its exclusion is very curious. Also, none of the volunteers observing the nesting boxes saw either a bluebird or a tree swallow eating a periodical cicada.
Tree swallow. K. Thomas, Public Domain
Although the egg production was very robust, the 2019 fledgling success rate for both bluebirds and tree swallows was quite low due to a very intensive impact of nest predators (black snakes, raccoons etc.). Fourteen bluebird nestlings and 20 tree swallow nestlings were eaten by nest predators. We realized that we had to do something to make our nest boxes safer and developed a plan to install predator guards for the 2020 nesting season.
Our house wren control was very effective for the 2019 season. Our diligence in removing the dummy nets discouraged the wren use of our nesting boxes. There were a total of 9 house wren nests but only 3 had eggs. There were only 4 house wren fledglings observed in 2019!
This year’s 2020 Cavity Nesting Team consists of 16 volunteers: Sharon Svitek and Lisa Kolodziejski take turns monitoring the boxes in and around the “High Meadow.” Dave and Kathy Brooke check the boxes around the “Bat House Meadow.” Dave Rizzo and Megan Concannon and Marianne Neal take turns monitoring the boxes in the fields near the Environmental Learning Center, and Paul Dudek and Donna Tolk check the boxes at the park entrance and around the soccer fields in the southern end of the park. The boxes around the “south” pond are checked by David and Kielie Ciuchtas and Denise Kelly. Patrick and Mardelle Kopnicky serve as resource people to help the new volunteers get adjusted to the program and to remind the rest of us about our working models and past observations, and Deborah and I are working this year as data compilers and reporters.
Our primary focus for 2020 is to see if our added predator guards reduced the predation pressure on our eggs and nestlings. Prior to this year 7 of our 29 boxes had predator guards. We added 15 new guards to our nesting boxes thanks to some volunteer work by an area boy scout and an immense amount of work by Dave Brooke (one of our volunteers) and a grant from Patrick and Mardelle Kopnicky. So, now 22 of our 29 boxes are protected.
So what have we seen so far this year?
Bluebird eggs. Photo by C. Urick
Bluebird nesting started right at the first of April! We had 6 nests the first weekend we went out to count and 7 more the second weekend! Bluebirds were mating and building nests very early and very rapidly! By the end of the first week in June we already had 22 bluebird nests which is well on the way to setting a new nesting record! There were 67 bluebird eggs observed but only 25 fledglings so far (there are still 21 bluebird eggs out there that haven’t hatched yet). There has been, though, a very high nest mortality rate for bluebird nestlings. We have observed 16 dead, bluebird nestlings and have had to remove them from the nests. The nestlings, apparently, were abandoned by their parents and either died of starvation or exposure/hypothermia.
The spring was quite cold (it snowed in mid-April!), and we speculate that food for the bluebird nestlings may have limited. Parent birds will abandon a nest if they cannot find enough food for the nestlings. Several area bluebird keepers have indicated that they have had to augment the food supplies for their nesting bluebirds.
Tree swallow nests. Photo by T. Schweitzer, Flickr
We have also seen 12 tree swallow nests with 29 eggs. As of the first week of June, none of these eggs have yet hatched. We have also found 2 dead adult tree swallows in the nesting boxes. There was no indication of the cause of death, although weather may have been a contributing factor.
Our house wren control program seems to be working. We have only had 2 completed house wren nests with 11 eggs. None of the eggs have yet hatched. Our predator controls seem to be working well. There has been no evidence of nest predation in any of our nesting boxes.
So, 2020, like almost all of our observation years, is unique! We are seeing robust bluebird and tree swallow nesting that has generated a record setting pace of egg production. But, we are also seeing the unforgiving impact of weather fluctuations with the nest deaths of a significant number of nestling birds.
More news later!