Tuesday, March 26, 2013


In this era of human domination of the world, a successful animal or plant may be defined as one that has adapted in some way to human presence, even benefits by it.

Cliff Swallows (Petrochelidon pyrrhonota) may be one of those animals. They build mud nests that they attach to vertical substrates overhung sufficiently to provide shelter from rain. They were able to evolve this nesting habit because of the widespread abundance of rocky canyons in western North America. Cliff Swallows are still abundant in that habitat.

The species was probably restricted to western canyonlands until barns and bridges built by settlers advancing across the Great Plains a few hundred years ago provided nesting substrates comparable to those provided by nature. The birds took advantage of these newly furnished nest sites and moved eastward, encountering more and more artificial cliffs and canyons as they went. Today they nest through much of the East as well, on buildings but mostly under big bridges across rivers.

Cliff Swallows are among the most social of land birds, with colonies of up to 3,500 pairs reported. Their closely packed nests extend over lengthy sections of cliff walls and can fill up the sides of barns and bridges wherever there is overhead shelter. Because they nest so densely, they are more subject than most birds to ectoparasites, especially swallow bugs and ticks, in their nests. Because of high chick mortality from high densities of these parasites, whole colonies are sometimes abandoned and the birds settle elsewhere.

Their other "enemies" include House Sparrows, which take over Cliff Swallow nests and even kill their young, and automobiles, which take their toll of birds nesting adjacent to roads. Recent research has shown that Cliff Swallows in such situations are evolving shorter wings, making them more maneuverable and less likely to be struck down by a car.

Swallow nests are somewhat messy, because after about a week old, the young defecate off the edge of the nest. A pile of poop can build up rapidly below the nest, sometimes even blocking the entrance! Swallows that nest where people live and work aren't always well loved because of this, and whole colonies are sometimes removed from highway overpasses because of the mess they make. Living around humans brings mixed benefits.

Dennis Paulson

Thursday, March 21, 2013


For some reason, purely by accident, it turns out that many of the world's swallow species now nest primarily in structures built by humans.

One of these species is the Violet-green Swallow (Tachycineta thalassina). Its breeding distribution lies mostly in extensive mountain ranges of the West. Originally the birds must have nested in natural crevices in trees and cliffs as well as holes excavated by woodpeckers, chickadees or nuthatches. Some of the swallows still nest in such places, and they remain common as montane birds.

However, fast forward to the early Twenty-first Century, and one of the best places to find Violet-greens is in our cities and suburbs. When I first moved to Seattle in 1968, there were nesting swallows all over the city. Some of them nested in houses, even more in commercial buildings, for example all over the University District. As I drove and walked around, I saw them all over the place. Any little opening into a building often had a pair of these beautiful birds nesting in it.

Nowadays they are still present in Seattle, but in reduced numbers. I have heard of no place where they are increasing and many neighborhoods from which they have disappeared. When I moved into a house in Maple Leaf, a wooded section of town, in 1991, I could dependably see and hear Violet-greens overhead on a daily basis each summer. A decade later, they were scarcely to be seen, and by 2010, they had disappeared from former haunts in many parts of the city.

A possible cause of this decline is the tidying up of our human habitat. The crevices in buildings that are used by Violet-greens and some other urban/suburban birds are presumably decreasing in number as people find them and seal them up. Who could argue with someone who wants to keep rats and mice out of their house!

Perhaps more significant, all swallows are aerial insectivores. In other words, their diet consists almost entirely of flying insects. Because of habitat destruction and pollution, primarily the use of insecticides on so many of our crops, we have brought about a widespread decline in such insects. Violet-green Swallows are decreasing generally as breeding birds in the Pacific states, although Rocky Mountain populations are doing well.

Even with local declines, swallows are thankfully still among our most common and visible birds.

Dennis Paulson

Tuesday, March 12, 2013


This isn't Capistrano, but our swallows are on their way back from their wintering grounds. The first to come are Tree Swallows (Tachycineta bicolor), which winter in southern US and Mexico. They probably winter the farthest north because they are programmed to migrate so early. In fact, a very few birds sometimes winter as far north as Ridgefield National Wildlife Refuge in southwestern Washington.

Tree Swallows are cavity nesters. But they can't excavate their own cavities like a woodpecker can, so they depend on natural cavities, old woodpecker holes, and nest boxes that we put up for them. They like to nest over water when they can, as predators that can climb trees to prey upon swallow eggs and young don't inhabit flooded wetlands.

Thus nesting cavities for them are quite limited, and there is fierce competition for them among swallow pairs. The earlier a male swallow returns to its breeding grounds, the more likely it will find an unoccupied cavity. Once a cavity is occupied, it's an uphill battle for the next bird that shows up to try to take it over, and so first come, first served.

Thus our Tree Swallows start trickling back to western Washington by late February, and the trickle becomes a river by the beginning of April. By that time, pretty much all available cavities are taken. Of course many of these birds continue on north, all the way to Alaska.

This surprisingly early arrival, well before our real spring, comes with a downside. The weather can be wretched at this time of year, cold and windy and rainy. Swallows are aerial insectivores, and their prey can be very hard to come by under such conditions. There is no doubt that in an especially bad spring, some of the birds succumb to starvation. There is no point in trying to raise a brood of young until conditions get better, so they don't attempt that until considerably later, some time in April or May.

Male and female Tree Swallows look the same, beautifully iridescent blue above and snow-white below, but with an interesting caveat. First-year males look just like older birds, but first-year females are recognizably different, dull brown on the back. There are scarcely any birds in the World with a unique first-year female, but the Tree Swallow is one. The adaptive significance of this first-year plumage is poorly understood.

Dennis Paulson