HOLE NESTERS

It’s late April, and bird migration is in full swing. Among the first birds to arrive are the hole nesters. Some birds make their own holes (primary cavity nesters), but there are others that nest in cavities but cannot excavate them themselves (secondary cavity nesters). The latter are always disadvantaged when it comes to finding a nest cavity, whether an abandoned woodpecker hole or a natural crevice. Basically, there aren’t as many nest sites as there are bird pairs. Even worse, competition is both within and between species for appropriate cavities.

One adaptive strategy in the intense competition for these holes is to be nonmigratory and to defend the nest site all year. It’s probable that the use of their holes for winter roosting has even favored resident status in primary cavity nesters. But interestingly, most of the secondary cavity nesters are migratory. So their best bet is to arrive as early as possible in migration and claim a cavity before anyone else can do so. Because of what must have been strong selection, many of the cavity nesters are among the earliest birds to arrive from the south in spring.

These include Tree (Tachycineta bicolor) and Violet-green (Tachycineta thalassina) Swallows, which arrive in migration in the Pacific Northwest before any of the other swallows. Tree Swallows normally arrive in western Washington in late February, in fact before almost all other migrants. They are competing for tree holes over water, and those are scarce indeed. Violet-greens arrive soon thereafter, looking for tree cavities in forests or niches in cliffs.

Some of these swallows occupy cavities in deserted buildings. That brings up the point that such birds will nest where they can, and human structures, deserted or not, can represent prime real estate if they have any openings into sheltered nest sites.

With our big brains, humans long ago figured out that if cavities were limited, a way to attract hole-nesting birds was to make nest boxes available to them. Thus the well-loved bluebirds have become recipients of so-called Bluebird Trails all over the country. Houses are put out in appropriate habitat at fairly close intervals, usually on fence posts, and they are rapidly accepted by bluebirds (Sialia spp.). Bluebirds and swallows seem always to reach higher breeding densities when provided with nest boxes. Much research on nesting biology has been carried out with the use of these boxes, and widespread declines in bluebird populations have been halted by their use.

There are many other secondary cavity nesters, including Wood Ducks, Buffleheads, goldeneyes, mergansers, kestrels, small owls, Myiarchus flycatchers, some chickadees and nuthatches, flying squirrels, red squirrels, and woodrats. When natural crevices don’t fill the bill, they all depend on the primary excavators, especially woodpeckers. Woodpeckers can be considered keystone species in the forest, as so many species depend on their presence.

Dennis Paulson

BUTTERFLIES IN WINTER

The majority of butterfly species in our region pass the winter in the larval stage (caterpillars), a smaller number as pupae (chrysalides), and rather few as eggs. But some of our common butterflies overwinter as adults.

They are the last generation of summer, and instead of just dying, as the great majority of adult butterflies and other insects do in autumn, they seek out a protected spot, for example under loose tree bark or in a rock pile, close their wings, and become dormant for the cold season. This can be as long as 4-5 months in the Pacific Northwest.

Adult winter dormancy is characteristic of the genus Nymphalis, butterflies with showy upperwings but very cryptic underwings. These adults are then the first butterflies of spring, even as early as February, as those with a hibernaculum in a sunny spot warm up enough to emerge and fly out to explore the world and, presumably, find a mate and breed.

The Mourning Cloak (Nymphalis antiopa) is the best-known among these butterflies, as its upperside is both spectacularly colored and unmistakable. Breeding on willows and many other common trees and shrubs, it is almost ubiquitous, although absent from dense forest. A freshly emerged Mourning Cloak is often the first sign of spring for a Northwest naturalist, rocketing through a clearing or resting on a sun-drenched log. Don't look for this species on flowers; it prefers tree sap, rotting fruit, and fresh dung!

Milbert’s Tortoiseshell (Nymphalis milberti) is another common species all over the Pacific Northwest. Its upperside is no less beautiful. This species breeds only on stinging nettle, but that plant is sufficiently common that Milbert’s are everywhere. They overwinter in the same sorts of places as Mourning Cloaks and emerge similarly early. Fresh individuals of both species have yellow bands on their wings that fade to white with age. That difference can be seen in the two photos here.

The California Tortoiseshell (Nymphalis californica), also known as Cal Tort to the butterfly cognoscenti, is similar in shape to the other two Nymphalis species but with still another dorsal pattern. It looks rather similar to other butterflies, for example several species of anglewings (Polygonia). The host plants are shrubs of the genus Ceanothus (deer bush, mountain balm), and these are most common from the Cascades east.

This species is of even more interest biologically than the other Nymphalis because of its population cycles. It is absent some years but present in prodigious numbers in others. If you ever drive on a mountain road in late summer and are dazzled by the sheer density of butterflies flying off the road and swirling around the car, they are likely to be this species.

Cal Torts that build up to these numbers undergo massive emigrations, pouring through the mountains and down to the coast, where they are seen flying south with other butterflies and dragonflies. The points of origin and destination of these migrating butterflies are unknown, but the movements may be a consequence of host plant availability, population pressures, and/or weather conditions. This would be a good species to study in depth.

Dennis Paulson

SOME SNAKES ARE SHARP, SOME ARE DULL

This is most apparent if you look at the ends of their tails.

The Sharp-tailed Snake (Contia tenuis) epitomizes the sharp end of the spectrum. The tip of its tail, really the last scale on the tail, is quite sharply pointed. This small and secretive snake is usually found under logs or rocks, and it presumably comes out at night, when it hunts for its primary prey, slugs. Oddly, one of the most commonly observed prey species is an introduced slug, and one wonders what they ate before the introduction!

It may be that the sharply pointed tail tip is handy for subduing slimy prey, but it also has been speculated that it may be used against predators (a snake may press it into the hand of a human captor) or even as an aid in burrowing. All pure speculation, of course, but we assume there must be some adaptive function of this prickly tail.

It has also been suggested that the banded pattern on the snake’s underside, sometimes exposed, may mimic the banded pattern of several millipede species of the area. As the millipedes are toxic and distasteful, some Sharp-tailed Snakes may escape predation by this mimicry.

Few people see this interesting snake, as its range is very restricted in the Pacific Northwest, and it appears to be active only for a relatively short time in spring and fall, when it is warm enough for reptile activity yet cool and moist enough so this small species isn’t stressed by low humidity. April and September are good months to look for sharp-tails by looking under objects on the ground that may serve as hiding places. Be sure to replace them carefully.

Rubber Boas (Charina bottae) are at the other end of the spectrum, with an extremely blunt tail for a snake. One of the consequences of this is that it is difficult to distinguish the head end from the tail end, and this may be the whole point, as it could be an effective anti-predator strategy. A snake captured by the wrong end might be more able to escape, and, if inclined, might even bite the predator. The latter seems unlikely, as these snakes are extremely docile, at least when picked up by a human.

Rubber Boas are much more common and widespread in the Northwest, usually associated with open conifer woodland. They are active all summer and take a wide variety of prey, mostly small mammals but also other vertebrates. They are active at night during much of the summer, but on cooler days especially they may be seen out and about. Otherwise, you look under rocks and logs, just as you would to find Sharp-tailed Snakes. A fortunate naturalist might find both of them together!

Dennis Paulson

SPRING IS THE TIME FOR GROUND SQUIRRELS

It’s hard to conceive of a species spending most of its life asleep, but that’s the case for the spring ground squirrels.

Ground squirrels feed on herbaceous vegetation, and many of them are active throughout the summer and then hibernate during the winter. Some ground squirrels, however, live in arid deserts or grasslands in which lush growing vegetation is present only during the spring or during a summer rainy season. These species are active only during that time, so they are active above ground for only about a third of the year.

Two species that exemplify this scenario are the Washington Ground Squirrel (Spermophilus washingtoni) and Piute Ground Squirrel (Spermophilus mollis). Both are small, short-tailed species. Piute Ground squirrels occur in a limited area of sagebrush steppe in southern Washington between the Cascades and the Columbia River. Washington Ground Squirrels occur east of that river in the Columbia Basin. Both species have declined substantially in recent years as their habitat has been lost to agriculture.

Belding's Ground Squirrel (Spermophilus beldingi) is a slightly larger and more southerly species of the high desert in eastern Oregon and northern California. It has adapted very well to agriculture, so it is still abundant, but its life history is much like that of the other two.

Washington Ground Squirrels have litters of about eight young. They are born in late winter and emerge from their burrows about a month after the adults. A visitor to a ground squirrel colony around the beginning of April is treated to the sight of playful young on all sides, while the adults go about the business of serious eating.

After a few months of activity, with an abundance of fresh salads on the daily menu, first the adults and then the juveniles retire back into their burrows and spend the summer in a state of torpor called aestivation. Aestivation then grades into the usual winter hibernation. The longest an individual of these species spends above ground is about four months. One adult Washington Ground Squirrel in captivity was dormant for 244 days.

Juveniles grow to adult weight in two months after leaving the burrow. All individuals begin to deposit fat 6-8 weeks after emerging and end up with lipids comprising about 65% of their body weight when they go into the burrow for good. They then live through the summer, fall and winter on the fat deposited during the spring.

High temperature and low humidity are stressful on any animal, and as there is no new growth on the plants they favor, the spring-active ground squirrels have no source of water during the summer; thus aestivation. Aestivation is an adaptation for survival in hot, dry climates, just as hibernation is an adaptation for survival in cold climates. These small animals use both of these strategies, surviving by the fine tuning of their seasonal activity to their climate. How will global warming affect them?

Dennis Paulson

THE BIRDS THINK IT’S SPRING

This is the time of year when our resident birds begin to sing. Already I have heard Anna’s Hummingbird, Bewick’s Wren, American Robin, Varied Thrush, European Starling, House Finch, Song Sparrow, and Dark-eyed Junco singing around my house. On the other hand, the vast majority of migrant species have not arrived yet, and those residents that are singing won’t be breeding until some time in April.

Why don’t all birds breed at the same time? Well, an easy explanation for raptors is that their incubation periods are long, so it behooves them to breed before many other species, so they have eggs hatching at a time when food resources are high. As it turns out, the eggs of the early breeders hatch at a time when many local mammals are weaning young.

Some species, for example raptors such as Barn and Great Horned Owls and Red-tailed Hawks, have already begun their breeding cycles. A Northwest pair of Red-tailed Hawks could have a complete clutch of three eggs laid by March 20. The first egg would hatch in mid April, and young would be ready to leave the nest by the end of May.

The bird-eating accipiter hawks (Cooper’s, Sharp-shinned) begin to breed later than the mammal eaters, because the supply of naïve young birds doesn’t really kick in until May or June. A Cooper’s Hawk pair that lays the first egg of a clutch of four on April 15 would expect the first young to hatch on May 20 and the first young to fledge about a month after that. By the end of May there are great numbers of young, naïve birds in the surrounding woodland that can provide prey for a family of Cooper’s Hawks.

A Barn Owl that laid her first of six to eight eggs on March 10 would see the first young hatch on April 10 and fledge in about two months. By hatching time, great numbers of young voles would have emerged from their grass nests to forage in their myriad runways, and the parents can find plenty of protein for their growing young. Furthermore, a just-fledged owlet should have no trouble detecting and catching those furry snacks.

Mammal-eating owls such as Barn and Great Horned breed early, but insect-eaters such as Western Screech and Burrowing Owls breed later for two reasons. First, incubation and fledging periods are shorter for smaller birds, so there is no reason to start so early. More importantly, their prey does not become active until ambient temperatures allow activity. So these owls are laying their eggs in April and even May.

A Burrowing Owl clutch of eight eggs might be completed on April 15, when incubation would ensue. The eggs would hatch around May 15, and the young would be ready to leave the burrow by the end of June, when insects and lizards abounded in its nesting territory.

So the eggs of the mammal eaters hatch about a month before the eggs of the bird and insect eaters, just as predicted. Not only the wonderful adaptations of living organisms, but also their exquisitely fine tuning, never fail to impress me.

Dennis Paulson

BIG BLACK BIRDS WITH BULGY BEAKS

These are the scoters, three species of ducks we are fortunate to see in the Pacific Northwest every winter. The males are black with bright head markings, the females mostly dark brown. They breed in Canada and Alaska and winter along most of the temperate coasts of the United States. They are mostly confined to salt water in the nonbreeding season, although wintering numbers on the Great Lakes have increased because of the proliferation of zebra mussels there.

The Surf Scoter (Melanitta perspicillata) is the most common of the three scoters wintering in the Pacific Northwest. While all three scoters include a variety of marine invertebrates in their diets, they seem to be bivalve specialists. The heavy bill of a Surf Scoter provides it with strength to pull mussels loose from their attachments to rocks and pilings. Accordingly, Surf Scoters are common where there are large beds of mussels, and that includes a lot of territory in this region.

Surf Scoter courtship displays are fun to watch. The male flies a short distance with loudly whistling wings, then slides to a stop on the water with both wings raised high. How could a female not be impressed? Like other ducks, they pair during the winter and fly back to the breeding grounds together. Most males probably don’t breed until they are two years old, and younger males can be distinguished in winter by less brightly colored bill and often smaller white head patches.

White-winged Scoters (Melanitta fusca) appear to be more locally distributed than their Surf cousins. They are most common in bays with sand substrates, where clams are much more common than mussels, surely indicating a food preference. The White-winged is a bit bigger than the Surf and is our largest duck. The big white wing patches are prominent in flight but may be hidden on a resting bird. Note the difference in feathering on the bill of the two species, useful when identifying juveniles, which look very similar.

The Black Scoter (Melanitta americana) is the least common and most locally distributed of the PNW scoters. Its head shape is more like that of other ducks, presumably indicating it is less extremely adapted for bivalve pulling. As in the Surf Scoter, the male wings whistle in flight.

Male scoters see their mates on a nest on the breeding lake and then head for the coast in what is called molt migration. Female Surf Scoters follow the males after they raise their young, but female White-wings undergo this molt on the breeding grounds. The molt migrants arrive at their destination and begin their body and wing molts. They are flightless during wing molt, so they need to be somewhere with abundant food resources for that period. After completing molt, many continue farther south along the coast.

Scoter migration is dramatic on the outer coast, where flock after flock of Surf Scoters pass by just offshore, heading south in September and October and north in April and May.  Smaller flocks of White-winged Scoters are interspersed, but you’ll have to look long and hard for the much smaller numbers of Black Scoters. Migrants of many other seabirds, in flocks and by themselves, add to the thrilling experience of a few hours spent on an outer-coast point.

Dennis Paulson

THE LARGEST SONGBIRD

This is not a bird that you think of when you think of songbirds, but in fact that’s exactly what a Common Raven (Corvus corax) is. Along with all the other members of its family, the Corvidae, it is a bona fide member of the suborder Oscines, the “songbirds” of the order Passeriformes.

Although no one would ever accuse a raven of singing, its vocalizations are varied. Most of the time we hear ravens giving their single or multiple somewhat musical croak, lasting for a second and with distinct harmonics. This is apparently a location call, making other ravens aware of the bird’s presence, and it is most commonly heard from birds moving over the landscape. That call is the epitome of raven sounds, but the species expresses a wide variety of other calls in varied circumstances. They are similar enough so that elaboration of their function has been difficult.

The Common Raven is also the widest-ranging corvid and a fine example of a successful species. It is distributed throughout the Far North, including Greenland and Iceland, and all across the northern hemisphere south to Nicaragua, northern Africa, northern India, and central China. Populations occur in all terrestrial habitats except rain forest.

Ravens are true omnivores, taking just about anything edible they can find in their environment. Because they are so large, they can be effective predators on a wide range of animals up to the size of pigeons. They spend much time hunting voles where those rodents are abundant, and a vole is just the size to be swallowed in a single gulp.

During breeding season, any bird with a visible nest is in danger from sharp-eyed ravens. Pairs fly over the tundra, and when a shorebird such as a Whimbrel spots a raven coming, it will leave its nest to begin mobbing actions. Unfortunately for the bird, the raven’s mate, off to the side, may have seen exactly where the nest was located.

Garbage dumps are favored hunting grounds for ravens, which may visit them in large numbers. A thorough analysis of their “prey” there is not for the faint of stomach. Similarly, well-traveled roads with their road kills furnish a linear cafeteria for ravens, especially in the morning when they can often be seen systematically searching their length. The guaranteed presence of carrion (everything dies eventually) may be one of the factors that allow ravens to live just about anywhere, from tundra to mountain forest to desert.

Ravens are well-known to accompany wolves and other predators on hunts for the chance that food scraps may await them. They have been suspected of purposefully attracting both wolves and human hunters to moose and caribou.

Ravens mate for life and are commonly seen in pairs. They are superb flyers, soaring like hawks or plummeting through the sky in spectacular dives. A typical raven antic is to do a half-roll or even a full barrel roll in flight, and they have been seen to fly upside-down for considerable distances.

As should be evident by now, ravens are near the top of the list of brainy birds. Corvid intelligence is well documented, and books on ravens by Bernd Heinrich (Mind of the Raven and Ravens in Winter) and John Marzluff (In the Company of Crows and Ravens and Dog Days, Raven Nights) present this documentation as fascinating reading.

Pet ravens are especially playful birds, especially young ones, and whether their varied antics (sliding downhill, hanging upside down, dropping and catching objects while in flight, pecking dogs on the tail) are all related to survival is debatable. (Don't try this yourself; ravens are a protected species). One certainty is that raven watching will never be boring.

Dennis Paulson