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

DIFFERENT MOLT STRATEGIES

Most birds molt their flight feathers in a fairly straightforward way, as explained in the last blog. The feathers on each wing molt sequentially, starting with the innermost primary and usually ending with the innermost secondary. This is called sequential wing molt.

But there are two other ways to do it.

Very large birds (above about 1 kilogram in weight) that need to be able to fly at all times, for example eagles, cormorants, and herons, can retain feathers for more than one year and molt only some of them each year. The largest among these take about three years to replace all the flight feathers. This molt strategy is called stepwise wing molt.

In many large birds, you can see the different feather generations in an outspread wing because the older feathers are more worn and thus a bit paler. These differences should be evident in this Turkey Vulture wing specimen from the Slater Museum collection.

The other strategy is simultaneous wing molt. If predation is the only reason they need to be able to fly, and birds can avoid predation, they can give up flight for a period of time and drop all their flight feathers simultaneously. This is the case in anhingas, ducks and geese, loons, grebes, and larger alcids, all of which can remain in the water during this period, out of range of their potential predators. Note this group includes the very large swans and geese, which perhaps could not fly very efficiently with gaps in their wings.

The female Mallard is typical of midsummer ducks with all flight feathers missing. The Common Murre also is in full wing molt, usually obvious because the wingtips can’t be seen above the tail.

This Anhinga has dropped all of its flight feathers and most of its tail feathers simultaneously and is in the process of growing them back.

Dennis Paulson