K. Thomas, Public Domain
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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. This past 2020 season has turned out to be just as productive and interesting as our previous years.
Bluebirds and tree swallows have very specific mating and nesting patterns. Bluebirds have an early nesting period in the spring (usually April and May) in which they produce about two thirds of their yearly eggs and fledglings. They then nest again later in the summer (usually July and August) producing the rest of their annual eggs and fledges. Tree swallows mate and nest in between these two bluebird nesting phases (usually in late May and June)! This temporal pattern reduces interspecific competition for nesting sites and allows the swallows to use many of the early bluebird nesting boxes. It also allows many of the later nesting bluebirds to use the emptied swallow nesting boxes!
In 2019, we saw that this cycle could be greatly altered by the weather. Bluebirds started nesting in March and then re-nested in June. 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 a good number of tree swallow nests (7 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 and an excellent source of large, flying insects on which they depend during their nesting seasons, so its exclusion is very curious.
K. Thomas, Public Domain
The 2019 fledgling success rate for both bluebirds and tree swallows, though, 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. To prevent these losses in 2020, we installed predator guards on 22 of our 29 nesting boxes prior to the start of the nesting season!
House wrens can also do a great deal of damage to other cavity nesting species, so we have been doing all we could to discourage these birds from utilizing our nesting boxes. In 2019, our diligence in removing the wren’s mating display, “dummy nets” was very effective in discouraging the wren’s use of our nesting boxes. There were a total of 9 house wren nests but only 3 had eggs and only 4 house wren fledges observed in 2019!
The 2020 Cavity Nesting Team consisted of 16 volunteers: Sharon Svitek and Lisa Kolodziejski took turns monitoring the boxes in and around the “High Meadow.” Dave and Kathy Brooke checked the boxes around the “Bat House Meadow.” Dave Rizzo and Megan Concannon and Marianne Neal took turns monitoring the boxes in the fields near the Environmental Learning Center, and Paul Dudek and Donna Tolk checked the boxes at the park entrance and around the soccer fields in the southern end of the park. The boxes around the “south” pond were checked by David and Kielie Ciuchtas and Denise Kelly. Patrick and Mardelle Kopnicky served 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 worked as data compilers and reporters.
Photo by D. Sillman
The 2020 season bluebird nesting started in early April! There were 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 third week of May we had 22 bluebird nests: a near record number! By the ninth week of our season (the week of May 29) we had counted 67 bluebird eggs and 25 fledges (with 21 bluebird eggs still unhatched). In the next two weeks, though, we observed a very high mortality rate for bluebird nestlings (16 dead nestlings in three different nests). There was no evidence of nest predators (two of the nests had predator guards and one did not) or physical trauma. 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 April average high and low temperatures were more typical of average March values!). This cold trend continued until the last week of May. We speculate that food for the bluebird nestlings may have limiting. Parent bluebirds will abandon a nest if they cannot find enough food for their nestlings. Several area bluebird keepers indicated that they had to augment the food supplies for their nesting bluebirds during this early nesting period. Since the intent of our study is to observe un-assisted nesting outcomes of our species, we decided not to provide additional sources of food for our birds.
Just before Memorial Day the weather changed. The high temperatures rose to above average levels and the precipitation declined. Starting around June 10, daily high temperatures regularly reached the upper 80’s or low to mid-90’s and there was very little rainfall.
Photo by D. Sillman
The bluebirds, possibly responding to this unusually warm weather, started their second seasonal nesting phase in mid-June, four or five weeks earlier than in a typical year. From mid-June to mid-July bluebirds built 6 new nests and laid 21 more eggs. There were 12 fledges from the “leftover,” 21 “early June” (first nesting period) egg cohort and then 18 from the 21 overlapping “late June” (second nesting period) cohort.
For the season, then, there were a total of 27 bluebird nests in 21 nesting boxes, 88 eggs, and 55 fledges (a 62.5% “success rate” of eggs to fledgling birds). As expected, two-thirds of the season’s eggs and fledglings were seen in the first nesting period, and the remaining third in the second nesting period. The big deviation from a typical year, though, was how quickly the second nesting period began after the first!
The tree swallows, as expected, inserted their nesting season in the irregular gap between the staggered ending of the earliest bluebird nesting period and the startup of the second. By early June there were 12 tree swallow nests with 29 eggs! Four more nests were then added by mid-June and 10 more eggs. Five of these eggs, though, were in a nesting box without a predator guard. These eggs were eaten by a nest predator (week of June 5) and represented our only nest predator losses for the season! The predator guards, then, were very effective at protecting the eggs and nestlings in our nesting boxes! In mid-May we also found 2 dead adult tree swallows in the nesting boxes. There was no indication of the cause of death, although the cold weather or lack of food may have been contributing factors.
Tree swallow at nesting box. Photo by D. Sillman
Tree swallow reproduction totals: 13 nests (9 that had eggs), 39 eggs and 14 fledges (a 41% “success ratio” of eggs to fledglings). The egg totals may be an undercount because of the difficulty in finding eggs in the feathered (and, often, parental bird occupied) nests. Swallow nesting began the second week of May (first eggs) and ended by the first week in July (last fledged nestling).
Our house wren control program seemed to be working up until the second week in July. There had only been two completed house wren nests up to that point with just 11 eggs. There was, though, a flurry of house wren nesting and egg laying throughout July especially in now unoccupied nesting boxes that had had either bluebird or tree swallow nests. The house wrens seemed to increase their mating activity as daily temperatures rose. We ended up with 7 house wren nests, 34 eggs and 20 fledges.
Only two boxes had house sparrow nests this year. Both nests were removed and destroyed.
Conclusions:
Predator guards are effective: we need to put them on all of our boxes.
Box placement is good: 28 out of 29 nesting boxes had nests. Six nesting boxes had two bluebird nests through the season. Three boxes had a bluebird nest and then a swallow nest.
Tree swallows continued to preferentially nest in the north part of the park. Only one tree swallow nest was located near the pond. The dense vegetation and lack of open water in the pond may be the reason the swallows avoid nesting near it.
Both bluebirds and tree swallows alter their nesting patterns with changing weather, but both continue to conform to a seasonal, temporal separation pattern probably to reduce direct, interspecific competition for nesting sites. Food supply (especially as affected by weather) seems to be the critical factor that determines the success or failure of both species’ reproductive efforts.
House wren nesting needs to be closely monitored and controlled especially in hot weather. If summer’s do get hotter (as predicted by Climate Change hypotheses), we speculate that house wrens will increase in numbers possibly at the expense of bluebirds and tree swallows.
And finally, a number of the nesting boxes are quite old and weathered. They need to be repaired or replaced before the 2021 season.