A LOT OF DRAGONFLIES

The Spiny Baskettail (Epitheca spinigera) is a dragonfly of the emerald family (Corduliidae) that occurs all the way across North America from Atlantic to Pacific. It is common in southern Canada and the northern tier of US states. It breeds in lakes, and apparently it can become very common in its optimal habitats.

It shares with numerous other species of Odonata the life-history trait of overwintering in the final larval instar and thus being ready to emerge as soon as its aquatic habitat warms sufficiently in spring. When this happens, apparently most of the larvae in the lake are ready to emerge at about the same time, so there are massive emergences over a period of a few days. I have yet to witness one of these (the larvae actually crawling out of the water, splitting their skins, and the adult that emerges flying away), but I did see the aftermath.

These scenes greeted us one sunny morning in June in southwestern Manitoba. Spiny Baskettails, hundreds and hundreds of them of both sexes, were hanging from every bare twig for several hundred meters along the entry road to Lake Audy in Riding Mountain National Park. The location is outside the Northwest, but the species occurs in the Northwest, so I consider it fair game.

Several things were of interest, besides the sheer staggering numbers of them. Very few were flying around, yet I would have thought there would be many prey insects in the air on this nice warm morning. Secondly, not a single one was resting on a leaf; every individual was hanging from a bare twig, even though it meant that when one was dislodged, it wasn’t that easy to find a new perching spot.

Several times we saw one try to land on the abdomen of a perched individual, but they were always shaken or buzzed off. All individuals were obviously immature, with reddish eyes. During sexual maturation, the eyes become glowing blue-green.

The sexes can readily be distinguished with a good view. Males have a slightly more slender abdomen base, with secondary genitalia projecting downward near the base, and three terminal appendages (two dorsal and one ventral). Females lack the bump at the base and have only the two dorsal appendages at the tip.

Mother Nature was showing off her profligacy in a big way here in an aspen woodland at the southern edge of the Canadian boreal forest.

Dennis Paulson

NAKED IN THE OCEAN

It’s time to home in on the extreme low tides we get in our local marine environment during daylight hours in the summer. Get to the coast an hour before the low and spend a couple of hours looking in the shallow water and especially in pools left by the receding tide. If the pool is rich enough, just sit down and enjoy watching the invertebrates and fish, if there are any, go about their business.

But don't think you have to shed your clothes; the nudity here involves the animals you are watching. Among the animals that inhabit such places are the nudibranchs (= naked gills). These are mollusks that appear to lack shells, something like seagoing slugs; many have prominent external gills. Like slugs, they are members of the class Gastropoda, which includes the snails. Having dispensed with a shell gives the nudibranchs both constraints and opportunities.

The primary constraint, of course, would seem to be a much greater risk of predation without a hard shell into which to retire when disturbed. Along with this is the risk of desiccation when exposed at low tides, so you won’t find many nudibranchs sitting around on exposed rocks like you do some of the snails.

These are mostly small animals, ranging from 1 cm (Rostanga) up to 20 cm (Peltodoris), but because of their conspicuousness and beauty, they are more admired and sought by tidepoolers than some of the other invertebrates that share their habitat.

Prominent anatomical features are two tentacle-like structures toward the front that are called rhizophores and are probably chemical detectors. Toward the rear there may be a ring of gills. And projections all along the upper surface are cerata, the bright colors of which are probably involved in predator deterrence, a warning coloration that goes along with distastefulness.

The lack of a shell has provided selective pressure for nudibranchs to develop other ways of protecting themselves from predators, and one of the most effective is the ability to secrete chemicals, including sulfuric acid in some cases, that make them inedible. They are surprisingly predator-free, and fish have been seen to spit them out. Anything as conspicuous as some of these are would seem to be predator resistant!

One of the opportunities for such animals comes in the form of being free to be predators themselves. They often specialize in animals that aren’t eaten by many other predators, thus assuring them a reliable food supply. Most of their prey items are sessile, fixed to the rocks on which they crawl, but a few are able to capture motile animals as well.

Acanthodoris eats colonial ascidians (tunicates) and bryozoans. Triopha and Janolus eat arborescent (branched) bryozoans. Doris and Peltodoris eat sponges, especially the crumb-of-bread sponge (Halichondra), and how they avoid being pierced by the sponge spicules hasn’t been determined. These two species are called sea lemons. The tiny Rostanga eats red sponges, on which it may be perfectly camouflaged. Hermissenda eats hydroids and incorporates their toxic nematocysts into its cerata, thus affording it protection from predators. Dirona eats everything, including not only the same prey as the others but also snails and crustaceans.

The species shown here were found at two localities on the north coast of Oregon in May. All are common in Pacific Northwest waters.

Dennis Paulson

THE CHORUS OF THE CHORUS FROGS

Early every year in the Pacific Northwest a familiar sound rings out, telling us it is spring whether it seems that way or not. this is the “song” or advertisement call of the male Pacific Chorus Frog, Pseudacris regilla.

Most people recognize the call immediately as the generic frog call of movies in decades past. As this is a common frog (or at least used to be) in Hollywood, California, its call was incorporated into many a movie that needed frog calls as ambience. In fact the “ribbit, ribbit” sound has become the stereotype of frog calls.

These frogs come out of their winter dormancy very early in spring, when, to paraphrase Robert Burns, a young frog’s fancy lightly turns to thoughts of love. The only way a frog has to express itself in such a situation is to call . . . . and call . . . . and call. And that they do, with surprising strength.

When a male frog feels these stirrings, he heads for the nearest pond or marsh, usually in the evening but sometimes even during the day. On arrival, he jumps in the water and swims to what he considers a good position. Only the frog knows why it is a good position, but it probably provides a place to hold onto the vegetation and a place where he can be easily seen as well as heard. Interestingly, it is often the same male that calls first each evening.

He begins to call: ribbit, ribbit, ribbit, a creaky two syllables that carries at least a hundred yards or more on a quiet night. Another frog heads for the pond, either because hopping downhill in a moist environment will lead to water or because it homes in on the first frog. The second frog begins to call, perfectly insinuating its calls between those of the first. The two may sound quite different, so we hear ribbit, rabbit, ribbit, rabbit, ribbit, rabbit.

A third male begins to call, amazingly also able to insert its calls into the soundscape so they can be heard as distinct: ribbit, rabbit, robbit, ribbit, rabbit, robbit . . . the pace is speeding up, and there is no room for a fourth frog, but that one calls anyway. As the chorus swells, the individual voices become less apparent, even though the structure may still be there, but a female approaching the pond can easily distinguish the individual voices.

Females apparently choose males based on the vigor of their songs, and as the evening progresses, more and more males acquire a mate. The male clasps the female and stays with her while she looks for a place to lay her eggs. She finds such a place, lays a cluster of eggs, and the male fertilizes them.

The eggs hatch in a few days, and the tadpoles grow quickly on a diet of plant matter. After a few months, they finally absorb their tails and grow a set of limbs and are then ready to leave the pond. If they survive the year, they will return the following spring, and the pond will resound again with the chorus frog chorus.

Dennis Paulson

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