With flashy colors, specialized bills and unusual feeding techniques, roseate spoonbills command attention
With bills submerged, in light that is painful to unshielded human eyes, a delegation of shockingly pink birds might be seen on any given winter day traversing a mangrove-edged flat on Florida Bay. The point of the exercise, of course, is to forage. But exactly what those bills are doing in that murky water has been until recently one of ornithology´s tantalizing mysteries.
Early settlers in the Keys confused this long-legged wading bird with flamingos, which sometimes wandered over from the West Indies. However, as Audubon found when he went looking for nonexistent flamingo nests in the 1830s, this is the roseate spoonbill, which Roger Tory Peterson called "one of the most breathtaking of the world´s weird birds."
Breathtaking because of its plumage, which is startling enough year-round but erupts in carmine and orange during the breeding season, accounting for the folk name "flame-bird."
Weird because of the 6-inch-long appendage that led to another colloquialism, "banjo-bill." Its full function was only conjecture until an aerospace engineer and a biologist recently attached a skull to a bicycle wheel and rewrote the chapter on spoonbill feeding tactics.
Peterson also remarked that the roseate spoonbill--a Neotropical species that can be found from Argentina to the Gulf Coast of the United States--was virtually wiped out during the plume-hunting era. "If it were not for the fact that some spoonbills still survived south of the border," he pointed out, "the species surely would have followed the passenger pigeon and the Carolina paroquet into the void of extinction." (Five other spoonbill species--all with pure white plumage--are found from Europe and Africa to Australia.)
Though they were easy targets, roseates weren´t usually hunted for the millinery trade, since the pink feathers quickly fade. Rather, the slaughter of egrets for their nuptial plumes caused spoonbills, which shared the colony sites, to desert their nests. The plume frenzy, which began in the late 1800s, ended around 1920 with only two dozen breeding pairs of roseate spoonbills surviving on the entire Gulf Coast, where once there were thousands. Today, the species´ status on the breeding islands of Texas and nearby Louisiana apparently is secure, due in large part to decades of protection from poachers by National Audubon Society wardens who patrolled colonial bird nesting islands.
Yet the roseate spoonbill´s presence today on Florida Bay seems almost as tenuous as it was in 1941. That summer, Robert Porter Allen, who practically invented the science of conservation ornithology, reported to the Audubon Society that the species´ recovery there was going poorly and he didn´t know why. The bird´s troubles, in retrospect, may have been due to the fact that spoonbills were still being shot for food in the late 1930s; back then their nesting ground was too remote for law enforcement. But by the mid-1950s the breeding population was steadily building toward a peak of 1,254 nests in 1972.
The present threat is an indirect one: the degradation of the mangrove ecosystem because of freshwater diversion from the Everglades to farms and burgeoning coastal cities--or straight into the Atlantic Ocean during the rainy season. "There has been a steady decline in spoonbill nests on Florida Bay," says Jerry Lorenz, a University of Miami graduate student who has been studying the problem for the last nine years. Lorenz estimates the present number at only 400 breeding pairs. Another 185 pairs nested last winter at National Audubon´s Florida Coastal Islands Sanctuary in Tampa Bay, and Lorenz says many probably were forced to use secondary habitat because of the poor conditions around Florida Bay. "This place is falling apart," he laments.
But we´re getting ahead of the remarkable spoonbill bill.
Think of a bird´s bill--a horny yet flexible extension of its jaws--as a "hand" its owner uses for nest-building, preening and sometimes as a weapon. Above all, the bird uses the bill to catch and handle food. And its shape and size often reflects specialized foraging tactics. For example, ibis, which are in the same family as spoonbills, have long, slender and decurved bills adapted for probing in mud. The roseate´s bill, on the other hand, is an inch or so wide just under its eyes, narrows in the middle, widens to 2 inches near the tip--and is decidedly flat.
Ibis and spoonbills are both tactile foragers: That is, the bill snaps shut as a reflex action when contact with prey stimulates nerves at the tip. The spoonbill usually feeds in water no higher than its knees, and it is able to breathe while doing so through nostril slits located high on its bill.
In uncommonly lyrical prose, Allen described how the bill sweeps "from side to side in wide semicircles, the mandibles slightly parted, the tips digging gingerly into the surface film of the soft bottom, beneath waters that are nearly always opaque. Delicate, sensitive to the small wrigglings and the darting, skittish movements of fish or prawn a quarter of an inch in length or of insects of even lesser dimensions, this keen, responsive instrument must serve as both eyes and hands." He was mostly right.
But engineer Daniel Wiehs and biologist Gadi Katzir, working in the early 1990s with a captive Eurasian spoonbill and a contraption that could have been invented by Rube Goldberg, discovered there is more to the spoonbill´s rhythmic bill-sweeping. The flattened bill, they found, creates mini-whirlpools that suck submerged prey items into the water column.
The Israeli scientists moved the captive bird to a fish-stocked pool in order to videotape bill-sweeping sequences. They marked the bill to measure immersion, and they found that the bird consistently kept the tip about one inch from the bottom as water depth increased. Then they rigged a spoonbill skull to a bicycle wheel to simulate the bird´s sweeping motion and test the suction forces created by the bill tip. The researchers reported that the mechanically operated bill displaced small snail shells up to four inches when it swept over the bottom. In nature, they point out, the disturbance probably triggers escape responses in some prey species. Animals lifted into the water during one sweep of the bill are grasped during the next sweep.
The spoonbill´s unusual feeding technique has side benefits. For one, the bill doesn´t graze the bottom, which might damage its sensitive tip. And the lift force created by the swirling water probably reduces the effort required to move the submerged bill forward as the bird works its way through the water.
On occasion, however, roseate spoonbills will abandon their sweeping tactics and dash after a school of fry that they spot on the surface, flapping their wings for balance as they try to jump ahead of the frantic fish. Analyses of the stomach contents of Florida Bay spoonbills have found that fish comprise as much as 85 percent of their diet, with shrimp and aquatic bugs making up most of the difference. The single most important prey item, Allen found, was the sheepshead minnow, a chubby and colorful little fish that is superabundant in spoonbill habitat.
Florida Bay spoonbills breed during the winter dry season--November to April--building sturdy stick nests on small, uninhabited islands. The courtship ceremony is elaborate, including a display flight in which the tongue is extended, causing the bird´s orange gular sac (or throat skin) to be conspicuously exposed. The paired birds cooperate in building the nest, the male fetching the sticks and the female assembling the nursery for two or three offspring.
The pantry for adult spoonbills is the mangrove shore of Everglades National Park. "There´s a whole series of little creeks that dry up in winter, concentrating fish in deeper holes," explains biologist Rich Paul, manager of Audubon´s Tampa Bay sanctuary complex. "That´s where spoonbills and ibis make their living." However, Lorenz has found that the mangrove ecosystem holds fewer fish than it once did because of higher salinity due to freshwater diversion. "The Everglades watershed has been cut in half," Lorenz says, "and Florida Bay is getting only a third of its historic flow."
A weaker prey base is especially devastating when winter storms occur and the parent birds are unable to find enough food. "For example, in 1998 the nestlings did fine; they moved out on the branches and began to fly around," explains Lorenz. "Then it rained for five straight days, the parents left, the fledglings couldn´t fend for themselves and most of them died." Heavy chick mortality in spoonbill colonies due to heavy rains is nothing new; Allen estimated average nest losses on Florida Bay at 35 percent. But, points out Lorenz, "The problem is that the birds no longer have the resources to withstand these events."
Help may come from the U.S. Army Corps of Engineers, which started ditching and draining the Everglades in 1947 and now has an eight-billion-dollar plan to restore some of the freshwater flow to its historic course. If approved by Congress, work could begin in 2002 and would take 20 years to complete.
And if Florida Bay´s roseate spoonbills can hang on until well into the next century, perhaps one day there will be as many flame-birds nesting on the mangrove islands and sweeping the flats as there were before the first plume-hunter fired a shotgun into a crowded rookery.
Field editor Les Line has observed roseate spoonbills in Florida Bay and Argentina.