The following article was first published in the Winter 2008 issue of the Bluebird Monitor, the journal of the Ohio Bluebird Society.
Bluebird Monitoring, Revisited
In the Spring 2001 issue of the Bluebird Monitor, page 6, I described my annual practice of counting active bluebird nests around May 15. By counting active bluebird nests during the first nest attempts of the season, I can compare values from each year to gauge winter survival rates. I have been raising bluebirds since 1968, and in recent decades, the numbers of available bluebird territories have remained relatively constant from year to year. I maintain and regularly monitor hundreds of nest boxes in two state parks, one wildlife area, an observatory, a theological school campus, and a waste water treatment plant, so my nest counts are large enough to offer accurate comparisons.
In my 2001 article, I explained that after comparing the 1999 mid-May nest count of 49 nests to 2000’s 61 nests, I could say that the breeding population had increased 24.5% between 1999 and 2000, which was very good news at the time.
It is time to revisit the census technique, and use it as a tool to discuss what has happened to the breeding populations of bluebirds during the last ten seasons. It is important to understand population trends so we can make effective management decisions on our bluebird trails based on accurate relationships, and not on unsupported assumptions. For example, interactions between Eastern Bluebirds and Tree Swallows deserve proper interpretation, especially after extreme weather has decimated bluebird populations.
Bluebirds Were Clobbered Before the 2003 Season
To promote better understanding of bluebird population histories in Central Ohio, I submit a chart showing the mid-May nest counts on my trails for the last decade of 1999 – 2008.
The chart to the right (Mid-May Bluebird Nests, Delaware County, Ohio) shows that, after 1999, numbers of breeding bluebirds remained constant for three years from 2000 through 2002, then the number of nests, or breeding pairs, was halved before the first round of the 2003 season. The breeding population had dropped from 62 pairs in 2002 to 31 in 2003. What happened?
When I first saw wild pink roses blooming in Delaware State Park in 2002, I looked forward to a healthy breeding population of bluebirds for the 2003 nesting season. Rose blossoms are indicators that wintering and migrating bluebirds can survive on rose hips, poison ivy berries, wild grapes and other dried morsels until warmer temperatures deliver insects. But ice during the 2002-2003 winter decimated the bluebird population.
Bluebirds cannot husk ice from berries. More than several winter storms started with ice, but the few icy incidents in Central Ohio were small compared to what happened in Southern Ohio. According to news accounts, thick layers of ice nearly deforested parts of some southern counties. These weather extremes must have sealed the fate of many bluebirds seeking shelter and berries throughout their winter range. I hunt squirrels in Shawnee State Forest in Scioto County, and after a huge ice storm swept across southern Ohio, the forest looked like it had been carpet-bombed; heavy ice had stripped limbs from most of the trees, turning trunks into bare poles. I feared for the bluebirds that had been wintering there.
While I can only imagine what happened south of my trails in 2003, my own nest boxes gave up twelve winter-killed bluebirds during spring cleaning. Five birds were fresh enough to weigh. A healthy adult bluebird weighs between 28 and 32 grams. The five dead birds weighed from 19.8 to 22.3 grams, which is 1/3 lighter than normal. Their breast muscles were nearly gone, a convincing sign that they had starved to death.
Something More than Ice?
In 2003, other states were reporting depleted bluebird numbers, some in regions where ice storms did not occur, forcing many to examine the possibility that West Nile Virus (WNV) may have been a cause. In Kentucky, 18 of 21 dead bluebirds were found with antibodies for WNV. Did the dead birds survive the virus and die of something else, or had the virus weakened them enough to affect their ability to survive the stresses of winter? Whether it was ice alone, or a combination with WNV, the 2003 nesting season on my trails started with one half as many bluebird pairs as the previous season.
Tree Swallows Scoop up Available Housing
Tree Swallows took advantage of the bluebirds’ bad luck in 2003. As an example, the entrance area of Delaware State Park is a mowed corridor that annually supports four bluebird pairs, but by May 1, 2003, only one box held a bluebird nest, while Tree Swallows were claiming the other three sites. For swallows in 2003, their only competitors were other swallows.
My trails have been saturated with Tree Swallows since the early 1980’s. At the beginning of any swallow nesting season, as many as one of every four of the aerial feeders is a floater, an unmated bird that will nest if it finds a suitable cavity. Swallow floaters are there to snatch up empty nest boxes after drastic declines in bluebird populations. Swallows claimed more nest boxes in 2003 after the bluebirds failed to show up, which enabled the aerial feeders to raise 13% more young than the previous year.
When bluebirds defend their traditional territories against swallows, their 50% weight advantage enables them to easily, and successfully, chase the gliding interlopers away, providing they have a suitable tree limb, wire, or pole above their nest to perch on. From an advantage point overlooking its nest box, a bluebird defender can drop to sweep a swallow from the face of its nest box, teaching the interloper not to come back. Of course, good defense strategies are learned with age, so a veteran bluebird is better at holding its nest site against swallows than a young bluebird nesting for the first time.
Building Back to Normal: 2003 – 2006
Bluebirds set about rebuilding their population in 2003, producing 219 fledglings. Even though there were half as many breeders than the year before, their production only lagged by 33.6% from 2002. The 2003 population accomplished this by raising more fledglings per pair than the 2002 birds. By the end of each of their seasons, pairs counted on May 15 in 2002 and 2003 raised 5.32 and 7.06 fledglings, respectively. Parent bluebirds reconstructing a diminished population have less competition from their own kind, resulting in larger territories with more food resources for each family, making clutches larger, and family maintenance more efficient, so parents produce more fledglings throughout the season. It’s nature’s way of rebuilding a thinned population.
The chart shows that the number of bluebird breeders increased from 2003 to 2004 by 38.7% (31 to 43 nests), then held steady for 2005 with 43 breeding pairs. I was very happy in 2006 when I tallied 58 nests in mid-May and realized that the bluebird population was nearly back to 61 and 62 nests recorded in 2000-2002. The celebration was short-lived as the 2006-2007 winter turned into a nightmare.
The Bleak Winter of 2006-2007
Wintering bluebirds, like their cousins, American Robins, enjoyed extremely warm temperatures during January 2007, then for weeks they were slammed with low temperatures, heavy snow, and ice. As a result, I found 45 dead bluebirds in 16 boxes on four trails, including 34 in Delaware State Park. My highest number of dead in a box was seven, counted in two nest chambers.
Spring-killed Tree Swallows
The carnage experienced during February 2007 spilled over into the spring as April had its share of record cold temperatures. When I monitored seven trails of more than 300 boxes for the first time, I found 18 dead swallows. I also found three boxes with mounds of four to eight LIVE swallows that were sharing body heat as they waited for warmer temperatures to launch flying insects. I gently closed the nest boxes without touching the birds to avoid stressing them. Days later, I returned to these same boxes to find two more dead birds.
As mid-May approached, I knew what to expect. The nest count revealed that bluebird breeding population had dropped from 2006’s 58 nests to a dismal 20, or a mere one-third (34.5%) of the previous year’s total. After two years of rebuilding, the population had been slashed back by an unforgiving winter and spring. Both bluebirds and swallows had been hit hard.
2008 and Beyond
The rebuilding process started anew and was partially successful as the 2008-bluebird nest count rose to 31 nests, a 55% increase, but a 50% deficit remained when compared to a high nest count of 62 in 2002. How long will it take for bluebird breeding population to recover?
Climatologists offer little hope as they agree that global warming is a reality. One promised symptom of global warming is erratic weather, a phenomenon that I am sick of seeing as I count fewer nests and more dead bluebirds.
In the meantime, when Tree Swallows move into nest boxes that historically housed bluebird families, ask yourself whether it is the fault of super-aggressive swallows, or lethal weather extremes. Well-thought out answers may lead to happier bluebirding, and a drop in your blood pressure.