May  2010


NESTING OF WADING BIRDS IS A BIO-INDICATOR

Cartoon of rookeryThe recovery of nesting wading birds will be an important bio-indicator of the success of continued efforts at Everglades restoration.

 

Cartoon of "rookery" of nesting wading birds. Can you identify the species?
They do not all usually nest together.

 

 

 

HISTORICALLY, 2,500,000 WADING BIRDS
LIVED IN THE EVERGLADES

Painting by Wendell Minor

Painting by Wendell Minor of hordes of wading birds flying to night roosts, as recalled by a Seminole storyteller

Past numbers of wading birds -- egrets, herons, spoonbills, ibises, and wood storks -- are only estimates based on descriptions of early explorers native Americans, and naturalists. The best estimate of numbers for south Florida is on the order of 2.5 million in the 1800's. The flocks hovering overhead so astonished observers that they could scarcely believe their eyes. The flocks reeked of fishy guano and sounded like a chorus of foghorns, whistles, and screeching babies.

 

WADING BIRD NUMBERS CRASHED IN THE 1900'S

Hunting for plumes for ladies' hats decimated egrets, herons, and spoonbills. In 1886 a birdwatcher strolled for an hour in a New York City shopping district and noted that 542 of 700 ladies' hats had wading bird feathers. An ounce of the most spectacular breeding bird feathers, "aigrettes", sold for more than an ounce of gold ($32). In 1886 the carnage of adult birds shot by plume hunters at breeding rookeries was five million birds; the chicks were left to starve. One Florida agent shipped 130,000 plumes.

Great egret spreading "aigrettes"
during courtship display.

Image by Lance Warley.

 

In the early 1900s public outrage and new laws stopped the plume hunting. The bird numbers then rebounded to perhaps 500,000. They showed only an erratic further recovery to a high of about 750,000 around 1935.

Great egret displaying "aigrettes"


MAJOR LOSS OF HABITAT CAUSED BIRD NUMBERS TO
DECLINE TO A LOW OF ABOUT 20,000 IN 1980

The area of wetland habitat declined starting in the 1880s as land speculators and flood control agencies ditched, drained, and diked the Everglades. By the mid 1900's only half the Everglades remained wet and many species became endangered. Since the loss of life from a 1928 hurricane and a massive flood in 1947, there was further draining for flood control and agriculture. So habitat area continued to decline.

Monster Dredge  Tomato Field
 Monster dredge by Wendell Minor  Tomato Field by Wendell Minor

Paintings  by Wendell Minor 

Before the Everglades were drained, the water level decreased steadily as water flowed from north to south. By 2005 Tamiami Trail and other "dams" had interrupted the flow, and  much of the remaining Everglades became unsuitable for wading birds to forage. Now most of the Water Conservation Areas, like our Refuge, are too dry and shallow in the north and too wet and deep in the south.

The Shark River Slough in the southern Everglades, the site of past super colonies of nesting wading birds, no longer gets enough fresh water flow. This area can no longer maintain the high production of fish, crayfish, shrimp, and aquatic insect food that wading birds need during the nesting season.

Reconstructed satellite view of former Everglades

Reconstructed satellite view of past Everglades with Shark River Slough outlet to Florida Bay to the south

Map of the natural Everglades. The shape and orientation of tree islands
show the direction of flow.

Feeding frenzy of herons and egrets in shrinking poolsThe historic Shark River Slough was a great site for nesting wading birds for several reasons. It offered an area with such a variety of water levels that wading birds could find drying-but-not-yet-dry marshes throughout their nesting season. Birds could start to forage in the shallowest wetlands at the edge of the marsh in December and progress inland to the deepest sloughs by March. As the depth of the water decreased, remaining pools supported as many as 600 fish per square meter, and attracted spectacular feeding frenzies. One reason for the especially high productivity was the freshwater-saltwater estuary interface at the south end of Shark River Slough.

A feeding frenzy of herons and egrets in shrinking pools.

 

In addition to dikes around the Water Conservation Areas, new roads like Tamiami Trail and Alligator Alley have stopped flow and acted like traps for migrating animals many of which are food for wading birds.

Taylor Alexander, a Florida naturalist, recalls the abundance of wildlife in the 1940s and the effects of the new Tamiami Trail. "Wading bird numbers defied our imagination , with 200 - 300 in each feeding flock. The road-kills made the road slippery and the smell was obnoxious. There were huge numbers of dead crayfish, lubber grasshoppers, frogs, turtles, snakes, marsh rabbits and even otters and wading birds."

By the 1960s wading bird numbers were as low as 25,000. Even the ibises and wood storks, which were never hunted because they did not have plumes, had declined drastically.

To restore flow to the southern Everglades we have started to remove some levees, to fill some canals, and to make a one mile bridge for the Tamiami Trail.

 

GOOD NESTING YEARS SHOW US WHAT WE NEED TO FIX
TO RESTORE HISTORIC NUMBERS OF WADING BIRDS

Detailed records of water conditions and nesting of wading birds have only been good enough for the last 30 years to begin to test multiple hypotheses of what makes a good nesting year.

The years 2001 and 2009 had record nesting with peak numbers the highest since the 1940's. As shown in a graph of seasonal hydroperiod for our Refuge, and true for all the Everglades, these years were very dry with slow, regular declines in water level from December to May or June.

Water depthsThe water depths for 2000 and 2009 were optimal for nesting with long dry seasons and slow, regular drop in water level. The water depths for 1995, 1998 and 2008 were too wet for successful nesting.  

In years when the slow drying was interrupted by increased water levels, birds failed to nest or abandoned nests. One cause of these reversals was natural El Niño rains as in 1998 and 2008. Another cause of reversals are management decisions to release water into marshes during the dry seasons, as in 1995.

Throughout all of the Everglades, the years 2001 and especially 2009 had excellent foraging conditions for all wading birds for the following inter-related reasons:

  1. The winter - spring dry season was very long.
  2.  Water levels dropped very slowly with no interruptions by rising water level.
  3.  Live prey became concentrated in fewer and shallower pools.
  4.  Nutrients were not too high to get thick vegetation in which prey could hide most of the time.
  5.  Water temperature was not too cold so fish were active and did not hide on the bottom.
  6.  Water temperature was not so hot that only tiny mosquito fish survived with the very low oxygen in the water.
  7.  Water was not so shallow near rookeries that pools dried when food needs were highest for feeding young.
  8.  Suitable foraging conditions foraging were closer and closer to rookeries so adults had less energy costs to feed their young as they grew and needed more food.

Next month we will explore why for some wading birds the water conditions have to be just right for successful nesting. We will see that for wood storks and white ibis nature has to be "just right". For these birds:

When it is good it is very very good
But when it is bad it is horrid

 

REVIEW QUESTIONS

a. The number of wading birds in The Everglades declined 100 - fold from the 1880s to the 1960s.

b. The major reason for decline of all species of wading birds
in The Everglades was the killing of breeding birds for
their plumes.

c. El Nino years with heavy winter - spring rains
have been years with the greatest nesting success of wading
birds in The Everglades.

d. To get very successful nesting of wading birds requires water conditions such as depth, temperature, and nutrients to be just right.