Signs of Summer 8: Cavity Nesting Team!

Photo by D. Sillman

Last year I wrote two blogs about our Cavity Nesting Team study up at Harrison Hills Park in northern Allegheny County (May 28 and October 29). We had a very good year with our bluebirds and tree swallows. We had 13 boxes that had bluebird nests with a total of 65 eggs and 48 fledglings (a 74% survival rate), and we had 9 boxes that had tree swallow nests and a total of 31 eggs and 22 fledglings (a 71% survival rate). Nine of our thirty nest boxes, though, did not have any nesting activity, so, using our locations of utilized boxes as a guide, we relocated seven of these inactive boxes to try to make them more attractive to cavity nesting bird species. We left two of the inactive boxes in their 2015 locations because they seemed to be in ideal spots for cavity nesting birds (we must acknowledge, however, that we were wrong about their ideal locations! (see discussion below)). Our overall criteria for relocation were quite straightforward: boxes too close together tended not have nests (so spread out the clumped together boxes) and boxes right on the edges of field (i.e. very close to surrounding woodlands) were not used (so move the boxes away from the extreme edges of our fields).

This year’s Cavity Nesting Team consists of eight volunteers: Deborah and I and Sharon Svitek take turns monitoring the boxes in and around the “High Meadow” area of the park. Patrick and Mardelle Kopnicky check the boxes around the “Bat House Meadow.” Chris Urik and Odessa Garlitz take turns monitoring the boxes at the park entrance and up in the field near the Environmental Learning Center, and Paul Dudek checks the boxes around the pond and soccer fields in the southern end of the park. Every box is checked each week, and then each observer enters their data into an on-line Google spreadsheet. Each week, Deborah compiles and distributes the growing data tables to each member of the team. Chris Urik also has made GPS maps of the park showing the precise location of each nesting box.

Photo by D. Sillman

As I talked about last year, 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 typically found in older, often dead trees and are frequently abandoned cavities that have been chiseled out by woodpeckers. Any site management plan that favors woodpeckers (allowing dead trees to remain in the forest and not managing the forest or manipulating it into an even aged stand) will favor cavity nesting bird species.

Nest boxes are artificial substitutes for these natural tree holes. Eastern bluebirds, in particular, came under a great deal of stress in the past century. The influx of the alien invasive English sparrows and European starlings along with the habitat spread of the nest parasite, the brown headed cowbird, were major reasons for the bluebird’s numerical and distributional decline throughout the twentieth century. Human destruction of nesting and feeding habitats also contributed to this decline. Human efforts to provide existing bluebird populations with suitable and secure nesting sites (“bluebird boxes”) have, however, been extremely successful in bringing this beautiful species back from the brink of possible extinction. The North American Breeding Bird Survey reports that since 1966 eastern bluebird populations have increased by nearly two percent a year! The Cornell Laboratory of Ornithology estimates the worldwide population of eastern bluebirds (80% of which spend at least some time in the United States) at a very robust 22 million individuals.

K. Thomas, Public Domain

We are just past the halfway point of the summer, but our nest box data is showing some very interesting trends. First and foremost, our strategy for moving the inactive nest boxes has been very successful. All seven boxes that we moved have had nesting activity this spring and summer! Five of them have had bluebird nests (and have generated 13 bluebird fledglings), one box had a successful chickadee nest (4 fledglings), and one had a tree swallow nest (with eggs, nestlings and fledglings that were too hidden in the nest materials to count). Further, four of these nests were then utilized by house wrens (although it seems these secondary nests did not produce any viable fledglings). The two inactive boxes from 2015 that we did not move (because they just seemed like PERFECTLY located boxes!) still did not have any nesting activity. We need to look at these two locations more closely to try to see why they were not utilized by any of our cavity nesting species! We need to learn to think more like bluebirds and swallows!

So far this summer 17 of our boxes have had bluebird nests (with 61 eggs and 45 fledglings). These numbers are almost equal to the totals from all of last year, and we are just coming into the second wave of the bluebird nesting and reproduction! We expect to significantly surpass last year’s egg and fledgling totals for bluebirds!

Tree swallow, K. Thomas, Public Domain

Our tree swallow numbers, though, have been less robust than last year. Only six boxes have had tree swallow nests and there have been only 9 confirmed fledglings (although two nests did have nestlings that we expected did fledge, but the nest construction prevented direct observation of the birds. Even so, the 9 fledglings (even plus the 6 or 7 that might have come from the concealed nests) is well below the 22 fledglings we observed in 2015. Tree swallows have been described as “income” breeders that base their timing of reproduction on

House wren, Photo by dfaulder, Wikimedia Commons

short-term rates of food intake near the time of breeding. Reduced clutch size in tree swallows is a strategic response to limited resource abundance. Since the primary food of nestlings are adult insects that have aquatic larvae possibly some factors (weather? water quality? something else? ) have reduced the abundance of these insects and the tree swallows have responded by curtailing reproduction. It is also possible that the increased abundance and nesting activity of the house wrens has affected the tree swallow nesting and reproduction efforts. House wrens are known to actively interfere with the nest box selection and nesting activity of tree swallows.

So the cavity nesting cycle continues! We will keep monitoring these boxes on through Labor Day. I will let you know our final counts and observations!