Follow up to my earlier Swallows-on-Cold-Days Post

I posted earlier this week about swallows feeding from the surface of water bodies during a cold and windy day.  In that post, I included a link to a report on a mass die off of swallows and intriguing research on some rapid evolution of swallow body and wing sizes by Mary Bomberger Brown at the University of Nebraska – Lincoln.  I intended to follow up with Mary to get more information, but she beat me to it and contacted me first!  I asked for permission to share what she told me, and she agreed.

This is what Mary had to add to my short blog post:

This sort of foraging behavior is fairly common in the swallows, especially at this time of year when the birds are transitioning from migration to nesting. All of the swallows that occur in Nebraska (Bank, Barn, Cliff, Purple Martin, Tree, and Violet-green) do it. They are picking insects off the surface of the water—insects just emerging as flighted adults from aquatic instars, surface species (e.g., water striders) or moribund adults floating on the surface. Usually swallows feed on concentrations of insects caught up in thermals, mass emergences or mating swarms. Those concentrations form in the sort of weather conditions that allow thermals to form (warm, sunny, high barometric pressure days). On cool, wet, cloudy days (low barometric pressure days) thermals don’t form, insects don’t swarm and hungry swallows are left to pick insects off the water surface. With their long wings swallows aren’t particularly well-designed for that type of acrobatic flight, so, energetically, that style of foraging is probably ‘net loss’ or ‘even sum’ for them, but better than not foraging at all. You can think of swallows as being flying barometric pressure indicators—low pressure, insects down low, so swallows down low, high pressure, insects up high, so swallows up high.

 

And, about the 1996 Cliff Swallow weather kill—Cliff Swallows (and probably most swallows) typically carry fat reserves sufficient to carry them for about 4 days without feeding, beyond that they starve and die. In the last week of May 1996, the weather was cold, wet, windy and miserable across the Great Plains. It was too cold for insects to emerge and/or fly. The swallows got wet and chilled when out trying to feed on insects that weren’t available…the only successful foraging they could do was picking insects off the water surface. The swallows survived for 4 days, but on the 5th day as much as two-thirds of the population died. The swallows that survived had shorter wing and tail feathers, larger skeletons and were perfectly bilaterally symmetrical, meaning they were efficient, acrobatic fliers that could carry larger fat reserves. The swallows that did not survive were just the opposite. In the years following the weather kill those swallows and their descendants have maintained the shorter feather lengths and larger skeletons (there was significant survival selection for those heritable traits).

 

The road kill study mentioned in the last paragraph of the blog showed that the wing feather lengths of Cliff Swallows nesting on bridges and road culverts declined significantly over the past 30 years. The shorter wing feathers made them more efficient, acrobatic fliers that could better avoid being hit by cars/trucks/SUVs/RVs and survive to reproduce, producing offspring who also had shorter wing feathers. The presence of humans on the landscape with their roads and vehicles was the cause of significant survival selection for that heritable trait…a demonstration of birds adapting to accommodate anthropogenic change in the environment. The two results (bad weather and road kill) are similar (shorter wing feathers leading to more efficient, acrobatic flight and survival selection), but with very different causes, one natural and one unnatural.

Many thanks to Mary for this great information!

Photo of the Week – February 7, 2014

When I woke up, the temperature was hovering around zero degrees F but there was almost no wind, so I decided to go for a sunrise prairie hike.  It was a beautiful morning, and there was a lot to see in the prairie, but I didn’t find much to photograph until I got down to the Platte River.  There, as I walked cautiously across the frozen surface of the river, I found a few small holes where flowing water was exposed…

Columns of ice along a patch of open water on the Platte River.  Hamilton County, Nebraska.

Columns of ice along a patch of open water on the Platte River. Hamilton County, Nebraska.

It looked like the splashing of the passing river by had created stalactites of ice around some of the uneven edges of the holes.  Over time, those stalactites had widened into wide-based columns that spanned the 3-4 inch gap between frozen surfaces.  Regardless of how it formed, it was sure attractive in the early morning sun.

A close up of the columns.

A close up of some of the columns.

The edges of the ice were rimmed by frost, as were some of the plants nearby.  Not that there was any need for the frivolous decoration – the ice was plenty attractive on its own.  Very carefully, I slid myself on my belly toward the edge of the ice hole until I heard the first small cracking sound.  That seemed like a good place to stop.  Then, lying on the frozen river and photographing ice, water, and frost, I thought about (as I often do) how glad I was that no one was around to watch me.

Frost-covered plants on a small island near the open water.

Frost-covered plants on a small sandbar near the open water.

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