River Bird Blog

Copyright 2007 by Richie Swanson

The towering double-trunk cottonwood seems a likely place to take a last-ditch listen for North America’s most steeply declining warbler, the cerulean. I pause during this field season’s final visit. A prothonotary warbler flashes in foliage instead, shaking a mulberry too big-looking for the tiny-gold bird to swallow. He tightens his bill and shuffles the berry. He drops it, dives down, darts up, and a brown-headed cowbird flaps its fledgling-wings, begging. The prothonotary feeds it, flits on, gleans a bug from a leaf and feeds a second cowbird fledgling.

Male prothonotaries weigh about 15 grams. Adult cowbirds may grow to nearly 50 grams and can deposit 30-40 eggs in other species’ nests per season. This prothonotary has evidently failed to reproduce this year. He will likely lose weight, raising the cowbirds. He may die from exhaustion.

The scene reflects changes in the Mississippi, the surrounding landscape and bird populations since European settlement. “Almost everything humans do in manipulating our environment is beneficial to cowbirds,” writes Lisa Petit, Smithsonian Migratory Bird Center. Before the 1800s brown-headed cowbirds bred only in habitat now virtually gone, short-grass prairies of the Midwest. They now feed at grain and livestock areas, pastures, farm fields, grassy areas, even urban birdfeeders coast-to-coast.

Throughout their range they find host species for their eggs in smaller forests that provide fewer acres for hosts to hide nests. Here, they parasitize tropical migrants and other species in floodplain forest 90% diminished in the Midwest, partly because the Mississippi and other channelized rivers inhibit sapling diversity and density and forest regeneration.

Smaller forests also provide easier access for nest predators such as crows, raccoons, opossums and common grackles. This morning grackle fledglings screech by the 100s, begging at nearby Osprey Marsh with 100s of European starling fledglings, an exotic species capable of evicting at least seven of the 20 bird species that depend upon holes in trees and stumps for nest-sites here.

Sunday, two great crested flycatchers perched outside a nest-hole used by red-headed woodpeckers June 2-17, amid swamp white oaks older than the river’s locks and dams. One great crested peered inside, perhaps investigating the cavity for a second brood. The red-headeds had mated on a nearby limb, had called from the hole. They disappeared in the middle of reproductive efforts.

Red-headeds have declined more than 50% since 1966, probably due to multiple causes. Two obvious hazards exist less than 150 yards from the oaks: creosote in abandoned railroad ties, also a highway with banks of insect-prey and traffic unforgiving to the species’ low take-offs.

Cerulean warblers sang amid the same oaks and tall cottonwoods during my 1998 breeding bird study, also during June 1993-1997. They winter in South American forests rapidly disappearing. They may lack stopover habitats necessary to cross the Gulf of Mexico during migrations.

Ceruleans once bred especially abundantly in old-growth forests of the Mississippi Alluvial Valley (the river valley south of the Ohio Valley). These forests are gone. Before clearing, the Mississippi Alluvial Valley may also have provided crucial habitat for the Bachman’s warbler, now probably extinct. The alluvial valley may still support 20% of the entire breeding population of the prothonotary warbler, a floodplain-forest specialist with only ten percent of its habitat remaining.

Though Aghaming apparently supported no ceruleans this year, prothonotaries arrived April 26 and established numerous nest-holes beside backwaters.

One male first sang by an open-topped stump near the oaks May 6. A female carried in wads of moss wider than her face May 26. A mirror showed one egg June 2 and five June 10, none dotted brown or oversized like cowbird eggs.

A female snuck into the stump with worms in her bill, zigzagging up a crack June 17. June 25 found the nest empty, perfectly in tact, not pulled apart as if by a raccoon, or littered by eggshells as if poked by a wren, though opossum-tracks crossed mine near the stump.

A fledgling pecked upturned roots July 1, a fluffy gray streak smudging its golden breast. It raised its head abruptly, and an adult female fluttered down immediately, and their bills met. An adult male fed a second fledgling in buttonbush and followed a third in nettles.

A red-shouldered hawk apparently guarded fledglings also, chasing a young Cooper’s hawk from an island. Two young belted kingfishers dove without success for prey, and two adults chased them back to fishing perches as if to insist they keep trying.

The week the federal government announced it will remove the bald eagle from its endangered species list, two eagle fledglings perched beside a nest established last year, the first at Aghaming in my 30 years of birding here.

Only 417 pairs of bald eagles existed in the Lower 48 in 1963. The ban on DDT, habitat protection and other policies enacted by humankind helped the species to recover to 11,040 pairs today.

The cerulean, prothonotary and Bachman’s warblers tell us we must also keep trying, solve bird-questions scientifically and let fly our own spirited solutions, finding ways to allow birds using two continents, not just one, to sustain themselves.

SOURCES

Hamel, P.B.  1995.  Bachman’s Warbler (Vermivora bachmanii).  In The Birds of North America, No. 150 (A. Poole and F. Gill, eds.).  Academy of Natural Sciences.  Philadelphia PA.  American Ornithologists’ Union.  Washington D.C.

Hamel, P.B.  Cerulean Warbler (Dendroica cerulea).  In the Birds of North America, No. 511 (A.Poole and F. Gill, eds.)   Philadelphia PA.

http://audubon2.org/webapp/watchlist/viewSpecies.jsp?id=176. 

Lowthier, Peter E.  1993.  Brown-headed Cowbird (Molorthus ater.)  In the Birds of North America, No. 47.  Academy of Natural Sciences.  Philadelphia PA.  American Ornithologists’ Union.  Washington D.C.

Petit, Lisa.  Brown-headed Cowbirds: From Buffalo Birds to Modern Scourge.  Fact sheet, Smithonsonian Migratory Bird Center. 

Petit, L.J.  1999.  Prothonotary Warbler (Protonotaria citrea).  In the Birds of North America, No. 408 (A. Poole and F. Gill, eds.)  Philadelphia PA.