Heavy Rains Challenge Tree Swallows

Tree Swallow Pair on Post - Photo Earl Harrison

As I left my bluebird trail of twenty nestboxes and two nest burrows at Alum Creek State Park in Delaware County on June 14, 2019, I had been greatly disappointed since I had found 18 dead Tree Swallow nestlings in five nests among 17 boxes claimed by swallows. Two of the five nests had families that were entirely wiped out. I knew that the state park had not sprayed for mosquitos, so my number one suspicion went to the weather and its effect on insect populations, mainly small, flying insects that are crucial in supporting Tree Swallows.

Nearly Fledged Tree Swallows

Four healthy Tree Swallows, soon to fledge.

That afternoon, my spirits were raised somewhat when I checked 57 boxes that make up my Panhandle Road Grid on the Delaware Wildlife Area where I found only one dead swallow nestling among 56 active nests.

On June 16, I received a communication from fellow bluebirder Darlene Sillick who reported that three rainy days with one day of cold temperatures grounded flying insects to cause greater than normal death rates among Tree Swallow nestlings. I agreed with Darlene’s explanations and during the following days more and more trail monitors were reporting similar bad news detailing swallow deaths.

With bad news on my mind, I went about checking my 166 boxes in Delaware State Park. During two days of monitoring, I counted 90 boxes that had been claimed by swallows, and I recorded 88 swallow nestlings that had died among 32 nests.

By the end of any nesting season, the influence of habitats becomes visible as I process and analyze my data from my data books. I transfer data from data books to master data sheets for each species that lists each nest’s box number, the date when its first egg is laid, along with its number of eggs, hatchlings and fledglings. Using master data sheets enables me to easily do the math to describe the season’s results.

During past years of normal weather patterns, I could, with confidence, announce that once eggs hatch, 90% of the young will live to fledge. Not anymore! Tree Swallows nesting on eight of my trails fledged 843 young in 2019 after 1371 eggs were laid. Of the season’s eggs, only 77.2% hatched, and only 61.4% of the original eggs developed to fledge young. The number related most to the adverse weather that grounded flying insects is the percent of hatchlings that grew to fledge: 79.5%. The more significant news is that more than two (20.5%) of every ten hatchlings failed to fly from their nest, a truly tragic reality.

The best overall production rate among the swallows was from nearly half of 45 boxes and nestjars making up the Alum Creek Prothonotary Warbler Project. The paired nest structures stand above lake water along the northwestern shore of Alum Creek Lake. Half of the boxes have 1-1/8 inch entrances to admit the prothonotaries while the remaining boxes have 1-3/8 and 1-1/2 inch holes for Tree Swallows and bluebirds. Swallows laid 109 eggs, 105 hatched and 88.5% of their hatchlings grew normally to fledge 89 young.

The worst reproduction rates for swallows surfaced in Delaware State Park where 538 eggs were laid, but only 52.6% of the eggs developed into 283 fledglings that represented 71.6% of the hatchlings.

I have maintained and monitored nestboxes in the park for 43 seasons since 1977. In 1977, the park was in early stages of succession from farmland into a forest. Since the beginning of bluebird trail management in the park, its maintenance crews have maintained mowed areas and used brush hogging to preserve bluebird and swallow habitats while House Wrens have profited the most as most of the park’s acres grows into a forest with many brushy edges. The wrens pay their rent by eating ticks.

So, how did House Wrens do during the wet 2019 after laying eggs on all but one of my trails? Wrens laid 1052 eggs, 77.7% hatched, and 73.7% of eggs developed to fledge 775. Once their eggs hatched, parent wrens successfully fledged 94.9% of their hatchlings. I have always called wrens “super bird,” so the past season was no surprise. I think the wetness of the season caused insects to climb up the brush to escape the saturated soil below. Such an event would make it easier for super bird to capture its prey.

…and bluebirds? They nested on six of my trails to lay 314 eggs, only 75.5% survived to hatch, and 73.7% of the original eggs fledged 216 young. (Super bird evicted 25 bluebird eggs among seven nests.) Most impressive is the fact that once bluebirds hatched their eggs, 94.9% grew to fledge. Bluebirds feed from the ground. They drop from a perch to snatch their prey, then fly from the ground with their mouthfuls. Again, maybe the heavy rains made the insects on the ground more visible, etc.

Heavy rains during the nesting seasons affect our birds’ ability to feed their young. But I am also concerned about what warmer weather does to our nesting birds, starting with winter weather. For instance, for the last three or more years, I have argued that when we can stand outside in February, and we are comfortable while not wearing a coat or jacket, some insects are wiggling out of hibernation. Then, a polar vortex, or other cold front, returns to wipe out or stress the exposed creatures to lower their populations prior to spring.

During recent seasons, Tree Swallow egg clutches are becoming smaller, and of course, so are the numbers of fledglings. Compared to past years, I am convinced that the number of insects have been lowered by warmer winter temperatures that made it easier for June rains to afflict havoc on our birds.

I will conclude with a very relevant observation. When I clean out a nestbox after its occupants fledge, I always remove the used nest, and before I return it to the surrounding environment, I methodically pull it apart to search for past histories, including evidence of blowfly infestations. For 2019, I found no blood-sucking blowfly larvae or pupae in any nests. No blowflies!

What might be good for the nesting birds, is a good indicator of what is going on out there in the world we share with the birds. I am convinced that recent weather trends are lowering insect populations that result in lowering bird populations. After good or bad news, continue to do your best for our birds. Happy bluebirding.