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

BALD EAGLE - HERO OR VILLAIN?

The Bald Eagle is the national symbol of the United States of America. It seems appropriate for a country to have such a majestic bird as a symbol. Long-lived, monogamous, good parent, characteristic of wild places, Bald Eagles excite awe and admiration wherever they fly.

There have been notable dissenters from this view, including Ben Franklin, in a letter to his daughter 20 June 1782: "For my own part I wish the Bald Eagle had not been chosen the Representative of our Country. He is a Bird of bad moral Character. He does not get his Living honestly. You may have seen him perched on some dead Tree near the River, where, too lazy to fish for himself, he watches the Labour of the Fishing Hawk; and when that diligent Bird has at length taken a Fish, and is bearing it to his Nest for the Support of his Mate and young Ones, the Bald Eagle pursues him and takes it from him."

Yes, Bald Eagles are inveterate kleptoparasites, robbing Ospreys and other raptors of their prey. Like all birds, they have terrific vision and are aware of what goes on all around them, even at some distance. Not even a swift and strong Peregrine Falcon can withstand the attack of an eagle determined to wrest a recently captured bird from it.

In the middle of the 20th Century, Bald Eagle populations were decimated by ingesting DDT along with the fish and fish-eating birds that they preyed on. DDT compromises calcium transport, and the eggs laid by the eagles, with inadequate calcium, were thin-shelled enough to crack under the weight of an incubating female. Reproductive success fell and populations declined along with it.

DDT was banned in the US in 1972, and eagle populations have been rebounding ever since, to levels greater than any previously documented. Their numbers have skyrocketed in particular in the Pacific Northwest, which must be optimal eagle country.

Unfortunately, the consequences of this are dire for some other bird species. Eagles are opportunists above all, and they have learned to make a living, at least in spring and summer, by hanging around bird colonies. With present eagle numbers, colonies of Great Blue Herons, Caspian Terns, and Common Murres on and near the coasts have been hit hard by these predators, sometimes just single birds taking advantage of the prey concentration.

The nesting birds have no way to withstand eagle predation, losing eggs, young and even adults to the predators. Even though eagles may eat a small percentage of the birds in a colony, their presence causes nesting to be disrupted to the point of complete colony abandonment. Because of this, numerous Great Blue Heron colonies have failed, and even huge colonies of thousands of murres and terns have been abandoned.

In the coming years, wildlife managers will have to figure out how to deal with this dilemma. Bald Eagles are not on the endangered species list any more, but they are still protected. The birds whose colonies they are destroying are also protected and of concern, and what should we do when one valued species affects another one so severely?

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