ODONATA EMERGENCE – A CHANGE IN VENUE

Dragonflies (including damselflies, both in the order Odonata) are aquatic as larvae and terrestrial (and aerial) as adults. These are very different environments, and organisms need different adaptations to be successful in each one.

Dragonflies, like amphibians, have successfully colonized these two different environments. Some amphibians remain in water, and their immature and mature stages are very similar. Others undergo a dramatic metamorphosis when they move from water to land, for example tadpoles to frogs.

In dragonflies the changes are even more dramatic. A dragonfly larva (nymph) is so different from an adult that you would never think they were the same organism. Each is perfectly adapted to its environment, but they must change radically to move from one to the other.

Most dragonfly larvae spend months, in some cases years, in the water. Very tiny when they hatch from the egg, they begin feeding on other small organisms immediately. With an inflexible exoskeleton, they have to molt to grow, so they enlarge each time they shed their cuticle and grow a new one. Each of these stages is called an instar. Larvae usually go through 10-12 instars before they are full size.

While in the last instar, they begin the amazing transformation of metamorphosis. Within the larval body, tissues are transformed from larval to adult tissues. All this happens while the larva continues to move around, feed, and try to avoid being eaten by some other predator. Finally, the change becomes such that the larva switches from aquatic to aerial respiration. It cannot feed any more by that time, and it heads for a place to emerge from the water.

The larva crawls up onto shore or onto a stem emerging from the water and begins its transformation. It anchors itself in place by its sharp claws. Soon a split appears in the cuticle of the thorax, and the adult within enlarges and begins to emerge. The thorax and then the head emerge, and the dragonfly rests in that position for some time, presumably waiting for muscles to firm up.

It then reaches forward and grabs its own skin or the stem in front of it and pulls itself completely out of the larva (the cast skin is called an exuvia). It is still more or less the shape of the larva, but then it begins to enlarge still more while still soft. First it pumps body fluids into the wings, which had been accordioned into very small wing pads. The wings get bigger and bigger, finally reaching full size.

The fluids then are pumped from the wings into the abdomen, which lengthens greatly. Eventually the fully developed wings open up, and the dragonfly remains that way for a while. Finally it lifts into the air and flies away. The entire process may take only a half hour in a small damselfly, up to several hours in a large dragonfly, but the result is the same. Free of the water at last, the dragonfly undertakes a completely different life from then on.

That life will be much shorter than the larval life, in the range of a week to a few months, but it will involve dispersal away from the water to feed and mature, then back to the water to mate and, for the females, to lay eggs. The cycle is complete.

Dennis Paulson

“TIS THE SEASON TO EAT DUCKLINGS,

Fa la la la la, they’re good for you.”

This just might be the spring song at the top of the Coyote Hit Parade. Ducks have been breeding for the past several months in the Pacific Northwest, and there is a steady supply of cute, fuzzy, edible ducklings. Mallards were first, and many of them have full-sized young now. They were followed by other species, including Gadwalls, the second most common breeding duck in western Washington.

Ducks lay clutches of around 8-10 eggs and incubate them for almost a month to hatching. Incubation begins when the last egg is laid, so the young all develop synchronously and hatch at about the same time. The female leads the ducklings from the nest off to a good wetland feeding area, watching carefully for predators.

She can warn her offspring to hide, but she can’t do much to protect them against the predatory mammals, birds, snakes, frogs and fish that might relish a duckling meal. A duckling might be a snack for a Coyote, a good lunch for a Mink, or an overstuffed belly for a Bullfrog.

The downy (cute) stage in a Mallard lasts about 25 days, and then they begin feathering out and enter their gawky “teenager” stage.  After another few weeks, they are fully feathered, and they can fly at around two months of age; most broods are abandoned by the female then or a bit before.

Males of most species of ducks desert their mates when incubation begins, but in city ducks, it seems that more and more males can be seen with their families, at least early in the season, and one wonders if there are genetic changes happening in these populations.

The males begin to molt out of their definitive plumage soon after leaving the females, changing to a female-like eclipse plumage and eventually molting all their flight feathers simultaneously. The Gadwall shown here is entering that plumage. After their brood has fledged, females also undergo a complete molt, although they don’t change plumage.

Meanwhile, predators are taking their toll. Rarely will you see a complete brood of ducklings. Instead, the numbers decrease week by week until there are often only a few left with any given female. Sometimes females combine broods, raising the level of predator awareness with two pairs of eyes, but the young still remain relatively unprotected.

In any case, all a pair has to do is raise two young successfully in their lifetimes to keep populations stable. Waterfowl populations as a whole are doing well, so those females must be doing something right! Perhaps it’s good that not all those ducklings survive, as wouldn’t we be knee-deep in ducks at some point?

Dennis Paulson

ACCIPITERS IN A MUSEUM

Sharp-shinned Hawk (Accipiter striatus), Cooper’s Hawk (A. cooperii) and Northern Goshawk (A. gentilis) are three hawks of different sizes that are basically quite similar to one another. They are at home in wooded country, where they nest and usually forage. However, during migration, all three can be seen in open country, and all three can be seen at any time flying overhead, above the forest canopy.

All three eat primarily birds, and all three have relatively short, rounded wings and relatively long tails, useful aerodynamically as they chase their prey through the vegetation. All fly with a rapid flapping flight, interspersed with short glides.

The two smaller species, Sharp-shinned and Cooper’s, have become relatively common in urban/suburban settings, where they find abundant bird life, especially where people concentrate the birds by feeding them. So the hawks that appear in people’s yards are often one of these two species. The larger Northern Goshawk tends to stay in large tracts of conifer forests in wilderness areas.

People have trouble distinguishing the species, especially the two smaller ones, and there have been volumes written about the identification of accipiter hawks in North America. These Slater Museum specimens are put forward to furnish would-be identifiers a better idea of the relative sizes of the sexes and species. The study skins were chosen to be representative.

The first photo shows immatures of both species, the smaller males on the left and females on the right for each species. The size differences are quite apparent, and a female Cooper’s is very much larger than a male Sharp-shinned, but there is a steady gradient from one sex and then one species to the next. You can see that a female Sharp-shinned is as close in size to a male Cooper’s as it is to a male Sharp-shinned.

Immatures of the two species differ on average in the markings on their underparts, with Cooper’s tending to have finer, more distinct streaks and Sharp-shinned broader, blurrier streaks, often with some barring on the sides. Nevertheless, you can see that there is much variation. The best mark, if there is any doubt about the size, is the tail, more graduated in Cooper’s (outermost feathers substantially shorter than the central ones).

The second photo shows the same species as adults, in this case with only one female Cooper’s. Note first the difference in the tail shape. There is really no difference in the ventral color pattern. The final photo shows the upper sides of the same birds. Note the difference in color in the sexes, the males with distinctly bluer upperparts, as well as the better-defined dark cap of the Cooper’s. The male Cooper’s on the left is growing in a new central rectrix.

Dennis Paulson

YAQUINA HEAD, PARADISE LOST?

The islands off Yaquina Head in Oregon have always been just right for nesting seabirds. Isolation from the mainland gives them safety from mammalian predators, a very important feature for colonial birds. Colonies of nesting birds would be sure to attract predators such as coyotes, foxes, raccoons, and other carnivorous mammals, but apparently most of these mammals are not inclined to swim to offshore islands, so birds that nest on these islands are relatively safe.

These are scenes from Yaquina Head on 6 May 2011. A great colony of Common Murres has been there for many years, occupying the tops of several of the islands. In addition, large numbers of Brandt’s Cormorants nest among the murres and on lower ledges, and Pelagic Cormorants and Pigeon Guillemots fill in other parts of the islands.

The colony was flourishing during our visit, probably several thousand murres and good numbers of the other species. But the colony faced threats like never before. Bald Eagles have continued to increase every year since the banning of DDT in North America, and they are proving very effective predators on colonial nesting birds. Once the birds began to lay eggs, eagles disturbed the colony daily, taking both adult and young murres as well as eggs, and the massive disturbances caused by their presence enabled crows, ravens and gulls to take additional eggs and young.

Unexpectedly, Brown Pelicans proved almost as great a threat. Immature birds, not old enough to return to breeding colonies, stayed at Yaquina Head through the summer and visited the murre colony regularly. Walking and flapping through the colony, they picked up dropped fish and then began picking up young birds to get them to disgorge their fish. If that wasn’t enough, they also began to swallow the chicks whole. Many more chicks fell from their nesting ledges and drowned in the surf. Further disturbance was caused by Turkey Vultures that visited the colony.

During 372 hours of monitoring the colony in 2011, observers from the Hatfield Marine Science Center recorded 186 disturbance events, during which 1034 eggs, 142 chicks, and 70 adult murres were taken. Depredation rates were three to ten times higher than in previous years. Researchers estimated that no more than 28% of murre pairs successfully raised chicks to fledging age. Cormorants, much more spread out and with larger chicks, appeared to suffer much less mortality.

Interestingly, not only Bald Eagles but also Brown Pelicans and Turkey Vultures are increasing in the Pacific Northwest in recent years, perhaps all as a result of the removal of DDT from the environment. A new balance may be struck as one set of species becomes less common as previously rare species increase. How will we protect the murres now that we have protected their predators?

Dennis Paulson

YES, WE HAVE VULTURES

We often think of vultures as big, ominous-looking birds sitting around the carcass of a lion-killed wildebeest in Kenya or lined up on rooftops next to a garbage dump in South America. But we have our own carrion-eating vulture in the Pacific Northwest, the Turkey Vulture (Cathartes aura).

Turkey Vultures occur throughout the tropical and temperate parts of the New World, from southern Canada to Tierra del Fuego. Those at the northern end of the range, in Canada and northern US, are migratory. Tremendous numbers of Turkey Vultures migrate through Mexico and Central America. The vulture migration over Panama in October has almost reached the status of a tourist attraction, as the birds wheel and climb above the high-rise towers of Panama City in the fall, along with Swainson’s and Broad-winged Hawks.

They return to Washington from their tropical wintering grounds in late March and April to occupy nest sites on cliff ledges or in broken-off hollow trees or other dark recesses, usually in remote areas well away from human disturbance. They lay 2 eggs that they incubate for about 40 days. The young remain in and near the nest for up to three months after hatching. Oddly, although commonly seen in the air, they are almost never seen at road kill, very different from the situation farther south.

These vultures are very often seen early in the morning perched with wings outspread, apparently to dry them out. Why this is so common in vultures and doesn't occur so much in other large soaring birds may be a mystery we never solve.

Turkey Vultures fly with their wings held up in a dihedral angle, which it turns out adds stability in turbulent air. They often fly close to the ground, so this is of great importance. As a wind current hits one side, that wing tilts up and the other down. As it tilts down, it approaches the horizontal, where lift is maximal, and that stabilizes it. Watch a vulture in windy conditions and see how it tilts from side to side.

Unlike most other vultures, Turkey Vultures have a well-developed olfactory sense. Field experiments have shown that they can find something as small as a dead mouse by its smell, even under a closed forest canopy. Watch a Turkey Vulture quartering on the wind, going back and forth as first one nostril and then the other picks up the smell of a carcass from upwind. By going back and forth as the smell gets stronger and weaker, they eventually home in on the spot.

At lower latitudes, where Black Vultures are abundant, they may use Turkey Vultures as their carcass finders, watching individuals of the other species and following them to the ground. The Blacks are also watching each other, so the stimulus from a descending bird spreads outward, probably for quite a distance. Blacks dominate Turkeys, so the Turkeys must be quick to take advantage of a “fresh” meal. In the forested tropics, the much rarer big white King Vultures may finally appear and scatter all the lesser birds.

Turkey Vultures have actually increased in the PNW in recent years, for reasons unknown. Are there more dead animals now? Are they being more successful on their wintering grounds? Were they reduced by DDT like so many other raptors and are still recovering? In any case, they are masters of the air and a pleasure to see in the sky.

Dennis Paulson