CASPIAN TERNS – FABULOUS BIRDS BUT NOT ALWAYS APPRECIATED

The Caspian Tern (Hydroprogne caspia) is the largest of the world’s terns and one of the most widely distributed. The size of a medium-sized gull, it is a most impressive bird, with its striking silvery-white plumage, black crown, and big red bill. Its vocalizations are no less impressive, a loud rasping call that one could imagine pterodactyls made on their breeding grounds.

Caspian Terns breed on islands in lakes or bays, usually in colonies of hundreds to thousands of birds. The largest Pacific Northwest colony is on East Sand Island, at the mouth of the Columbia River, with about 10,000 pairs. This may be the largest colony in the world. The first eggs are laid in mid April, the first hatching about a month later and the first fledging about a month after that. Thousands of birds are still present in late July, but the colony empties shortly after that.

Most pairs lay three eggs, and when they hatch both adults are kept busy bringing in fish from surrounding waters. They fly well above the water, up to 10 meters or more, and dive when they see a fish near the surface, submerging their body completely. If they succeed, they head for the colony. The young have a high-pitched call, like most young birds, and in and around a colony the constant calling back and forth between parents and offspring brings to light the meaning of the word “cacophony.”

Terns are all fish-eaters, and big colonies of Caspians can only thrive where there are a lot of fish in the nearby waters. The Lower Columbia River is such a place, rich in many kinds of fish at the transition from fresh to salt water. Among them are salmon of several species. After their early growth in fresh water, salmonid smolts descend the river to spend some years in salt water before returning to spawn. It has been estimated that 100 million smolts come downriver each year, most of them released from hatcheries.

The terns, and other fish-eating birds nesting in the same area, are there to receive them. It has also been estimated that the tern colony on East Sand Island accounts for predation on about 5 million of these smolts. Although this is only 5% of the estimated smolt run, it is enough to greatly concern wildlife management personnel in both Washington and Oregon.

In fact, concern seems sufficiently great that the terns have been systematically persecuted. Over the past few decades, Caspian Tern colonies have formed at numerous sites along the Washington coast, and at each of them the birds have been hazed until the colony disbanded. Because they have been chased away from islands, they have tried to nest on rooftops and other humanmade structures, but each time they are discovered, they are soon displaced.

Because a few of the colonies on Columbia River islands are thriving, there are enough excess terns that they are still to be seen and heard flying over all of our coastal waters, but many of them fail to breed because they are not allowed to. Salmonid fishes are of great importance to the Pacific Northwest economy, and Caspian Terns are not; the equation does not favor the terns.

POSTSCRIPT (added 23 June 2011)

The largest PNW tern colony, on Sand Island at the mouth of the Columbia, has been severely disrupted by predators, especially Bald Eagles, and will probably fail this year. Fortunately, the terns are long-lived and will presumably breed next year, but the eagles are proving to be real villains in this case, disrupting colonies of terns and other seabirds severely enough that they pose a long-term threat to the populations of these birds. Anthropogenic changes may have made the situation worse, with a decline in other eagle prey and a reduction of the seabird colonies to few sites.

Dennis Paulson

IT’S SHOREBIRD TIME AGAIN

Every year in April and May the Pacific Northwest experiences a mad rush of thousands and thousands of shorebirds—sandpipers and plovers—on their way north to their Arctic and Subarctic nesting grounds.

This is nowhere more obvious than around two big coastal estuaries, Grays Harbor and Willapa Bay. These estuaries are extremely productive of the invertebrates that the shorebirds eat. Prey animals are present at very high densities in the mud and sand of these estuaries, for example about 10,000/square meter at Bottle Beach, on the south side of Grays Harbor, one of the most productive areas.

This seems astronomical but pales in comparison with 100,000 individuals/square meter of one species of amphipod in the Bay of Fundy. Nevertheless, it is sufficient density to support the tens of thousands of shorebirds that visit the area for up to a month each spring. Individual birds stay only a few days, fattening up for a flight that may be only 200 kilometers to Roberts Bank in British Columbia or as much as 900 kilometers to the Copper River delta in Alaska, then on to arctic breeding grounds.

The invertebrates that fuel these birds are primarily amphipods and polychaete worms. The amphipods are about 5-6 mm long, and some of the polychaetes are in the same size range. Other polychaetes are called “thin worms,” up to 30 cm long but only 1 mm in diameter! They can be in such high densities as to almost bind the mud together.

The three most abundant species in spring migration are Western Sandpiper, Dunlin, and Short-billed Dowitcher, in descending order. Most of the big flocks you see will include these species. There are many other species present as well, and searching them out gives the observer variety as well as spectacle. Other common species include Black-bellied and Semipalmated Plovers, Greater Yellowlegs, Whimbrel, Marbled Godwit, Ruddy Turnstone, Red Knot, Sanderling, and Least Sandpiper, with numerous others even less common but out there somewhere.

No migration is evident at the beginning of April, but by the middle of the month, Greater Yellowlegs and Short-billed Dowitchers have arrived in numbers. During the last two weeks of the month, all the species move in, most of them in full breeding plumage, and peak right around the first of May. Numbers fall off after that but are still impressive through mid May, and some of the later migrants are present until the end of that month. Species that peak late include Red Knots and Long-billed Dowitchers.

Herb Wilson of Colby College, Maine, found that these hordes of shorebirds had little effect on the invertebrates on which they fed. He erected “exclusion cages” at Bottle Beach, meter-square cages that kept shorebirds out, and after the migration compared the numbers of invertebrates under each cage with the original numbers there and the numbers just outside. He found no fewer invertebrates outside than inside; thus the numbers had not been reduced by the birds.

The spectacle can be seen in feeding areas at lower tides and at roosts at higher tides. The ocean beach itself has throngs of shorebirds on peak migration days, but it is discouraging to see the high levels of human activities (joggers, horseback riders, dog runners, mopeds, pickup trucks) that disturb each flock again and again on a busy weekend day. The birds get back to feeding immediately after each disturbance, and we can only hope that they are able to take in enough nourishment to make it to their next destination on time.

Dennis Paulson

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