(Click here to listen to an audio version of this blog!)
Over the past five years I have written a number of articles and posts about our Cavity Nesting Team study up at Harrison Hills Park in northern Allegheny County. This year we have twenty-nine nesting boxes scattered across the park, and the Team started our 2019 monitoring on March 30. We have built up a steady accumulation of observations and hypotheses during the years of our study. 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 observed a record number of bluebird nests (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 the large number of lost or undeveloped eggs reduced the overall egg production for the season to the second lowest total of the previous 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 (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 (63%).
Why did we see an apparent breakdown in the nesting and fledging success of our bluebirds? The weather last 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.
This year’s Cavity Nesting Team consists of 17 volunteers: Deborah and I and Sharon Svitek take turns monitoring the boxes in and around the “High Meadow.” Dave and Kathy Brooke and Lisa Kolodziejski 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 Maureen and Dave Sagrati and David and Kielie Ciuchtas, and 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. Data from our observations are uploaded to an on-line Google spreadsheet, and each week I summarize and distribute the accumulating data to all the members of the team.
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. The lack of the these older trees in most forests has led to a “housing crisis” for these cavity nesting species. Nest boxes, of course, are artificial substitutes for these natural tree holes.
So what have we seen so far this year?
We saw our first two blue bird nests on March 29: one in a nesting box near the park’s Environmental Learning Center and the other in the High Meadow in the northern section of the park. These ‘first nests” of 2019 were seen three weeks earlier than the “first nest” of 2018! The first 2019 eggs were seen on April 12 (16 days earlier than the “first eggs” of 2018). By mid-April we had 8 bluebird nests scattered throughout the park and 20 eggs. By the end of May this first cycle of bluebird nesting and reproduction had generated 10 nests, 38 eggs, 33 nestlings, and 33 fledglings. A second wave of bluebird nesting then began around the first of June. Nine new nests were build in previously unused boxes (31 eggs, 3 nestlings so far), and four new nests were built in previously used bluebird boxes (20 eggs and 5 nestlings so far).
We have observed, then, a very robust production of eggs by bluebirds (89 eggs so far) and a very high rate of success of the transitions of egg-to-nestlings and nestlings-to-fledglings. In previous years the second nesting cycle of the bluebirds did not happen until mid to late July! We may see this year, then a third nesting cycle of these bluebirds. The 89 observed eggs already exceeds the largest seasonal egg total from any of our previous study years (83 eggs were observed in the 2016 season)!
Coincident with the second bluebird nesting, tree swallows also began building nests in our nesting boxes. In all of our previous nest box observation seasons tree swallows did not start their nest building until mid to late-June. The swallows, then are, like the bluebirds, on an accelerated time table in 2019 (3 to 4 weeks ahead of all previous years). We had seven tree swallow nests built by late May, and only one of these nests was in the south part of the park (near the pond).
In 2015, all 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. Only one box (Box V here in 2019) of these pond-area nesting boxes has subsequently been used by tree swallows! We are still uncertain why the swallows are avoiding these seemingly optimal nesting sites! We speculate that the vegetative structure of the pond has changed over the years making it less desirable for tree swallow use or making it less productive for the aquatic insects upon which they preferentially feed.
A total of 35 eggs were observed in these seven tree swallow nests and 27 nestlings. Nest predators, however, raided two of the nestling-inhabited nests up in the Bat House Meadow and ate 10 nestlings during the first week of June. Of the 17 remaining nestlings, 3 have fledged and 14 are still in their nests.
Our strategy to control house wrens and reduce their impacts (egg and nestling destruction) on bluebird and tree swallow nests was developed during the 2017 and 2018 monitoring season. Male house wrens make “dummy nests” as part of their mating displays. Our strategy was to remove these partial nests whenever we came across them and thus keep the wrens in a constant state of display rather than actual reproduction. The 34 house wren eggs of 2016 and 2017 were reduced to 13 in 2018 (and total number of house wren fledges went from 23 in 2016 to 4 in 2018). Thus far this year there has been only one successful house wren nest (6 eggs) but all 6 eggs failed to hatch.
We had another house sparrow nest this year. The nest (and its 3 eggs), though, were destroyed by our nest box monitors. House sparrows are an extremely destructive, alien-invasive species. Their nests and eggs are not protected by the Migratory Bird Act, and their destruction was more than justified.
Why are the bluebird and tree swallow nesting cycles starting so much earlier this year? Why are the bluebirds and the tree swallows producing such large numbers of eggs? Could it have something to do with the spectacular, local emergence of the periodical cicadas? Both bluebirds and tree swallows feed their nestlings invertebrate prey that is much smaller than the robust periodical cicadas. I have not seen any tree swallows catching and eating cicadas and none of the Team members have seen a bluebird eating a cicada. Possibly, though, the cicadas are having an indirect impact on the abundance of the smaller prey. Bluebirds (according to Benedict Pinkowski in his March 1978 Wilson Bulletin paper) primarily feed their nestlings butterfly and moth adults and larvae, grasshoppers, crickets, beetles and earthworms, and tree swallows (according to the Cornell Laboratory of Ornithology and the Tree Swallow Project) primarily feed their nestlings flies, leafhoppers, ants, wasps, bees, beetles and, when available, dragonflies. Could these prey items be more abundant if other competing avian predators are feeding on cicadas?
Next week: unexpected interactions of periodical cicadas and birds!!