July 2010

GOLDILOCKS CONDITIONS
FOR WOOD STORKS

 

Wood stork feeds half-grown chick

Wood storks feed youngGoldilocks, in Goldilocks and the Three Bears, sought bed softness that was "just right." Similarly wood storks need water conditions that are "just right" to nest successfully. Water cannot be too shallow or too deep. Conditions cannot be too dry or too rainy. And aquatic vegetation cannot be too dense or too sparse. All of these affect the numbers of live prey that wood storks can catch to feed their young.

Because water conditions were "just right," 2008-2009 was the best year for wood stork nesting since the 1930's. Review last month's column, Nesting of Wading Birds is a Bio-indicator  to read why.

Water-level graph

In contrast, the El Niño rainy springs of 1997-1998 and 2009-2010 were the worst years on record for wood stork nesting. Rainfall was very high and the rainy season was so long that the dry season was almost non-existent. In Corkscrew Sanctuary wood storks did not even start to nest. Elsewhere storks that started to nest did not do so until March, and none of them fledged young.

Water level graph of years that were too wet (1997-1998 and 2009-2010), a good year (2007-2008) and a "just right"
Goldilocks year (2008-2009)

 

 




Nesting wood stork numbers for
the best and worst years for which we have good records.

Very Dry Nov - May Very Wet Nov - May
 2000-2001 2,200 nests,
most fledged
1997-1998 150 nests,
all abandoned
         
 2008-2009 6,542 nests,
most fledged
  2009-2010 Few nests,
all abandoned


WATER CONDITIONS ARE MORE CRITICAL
FOR TACTILE FEEDERS THAN FOR VISUAL FEEDERS

Tactile feeders, like ibis and wood storks, find it more difficult to catch live food than visual feeders, like herons and egrets. Researchers have measured the difficulty of feeding. They put known numbers of fish in a pond and waited for birds to come and feed. The tactile feeders left the pond first and the visual feeders continued to catch fish. So, we say that the tactile feeders give up when the fish density is still high enough for visual feeders to see and catch food. Eventually the fish density gets low enough that even visual feeders give up.

Joel Brown has called animals that give up when food density is high "cream skimmers" and animals that do not give up until food density is low "crumb pickers." See my earlier columns Herons and Egrets and Animal Aggregations for how this relates to wading birds.


WOOD STORKS AND WHITE IBIS
ARE BOTH "CREAM SKIMMERS"

We hypothesize that wood storks need higher prey densities to nest than white ibis because wood stork tactile feeding is less efficient. Wood storks use their open bill to grope for fish that are scared up as they stir the water with one foot. And they predominately catch fish when their bill snaps shut in 25 thousandths of a second. White ibis probe for food. and their bill tip opens and grabs prey. With this method of feeding they catch fish in vegetation, crayfish in burrows, and even cockroaches in lawns. But the wood stork catches fish only where its beak can move freely in the water.

Raising Young is a Huge Problem for Wood Storks

First, the wood stork has the longest incubation time and time from hatching to independence of young of 110 - 150 days to fledging. So it needs to start nesting as early as December to complete its nesting cycle. In the 1950's most storks started to nest in November and December. Nesting was successful except for one year when nesting started in March. Since the 1980's nesting was highly successful in only two years when nesting started in January.

Second, the wood stork needs the most food with the highest nutrition (i.e. 1 - 4 inch fish) as nestlings near fledging. They soar like turkey vultures and find food up to 50 miles away but need food even more concentrated and closer to the colony as the young near fledging size.

This sequence of images is of wood storks as they forage, feed early nestlings, and young wait in trees for adults to feed them before fledging.


  Wood storks foraging
Group of wood storks foraging with open bills
  Wood storks with nestlings Stork fledglings fishing
  Wood stork feeding very young chick
photo: Mike Godwin
Group of wood stork young nearly ready to fledge

I expect that wood storks may take a year to learn how to feed efficiently, but I have found no data to test this hypothesis.

Two fledgling wood storks (fuzzy heads) learning to feed next to two adults

Stork fledglings fishing

 

White Ibis Are More Successful Than Wood Storks

White ibis are also tactile feeders but are much less constrained than wood storks. Consequently, white ibis have had more years of successful nesting than wood storks. And in the best years white ibis total numbers and numbers of rookeries have been higher than any other species.

White ibis are more successful than wood storks for several reasons.

White ibis are smaller birds and so they need smaller and fewer prey. This is reflected by their shorter nesting cycle of 70 - 80 days.

White ibis have both a broader diet and wider habitat use. White ibis probe for food, especially crayfish, in the substrate and under and around obstacles. They also catch fish in open water and in dense vegetation. Furthermore they can supplement their diet by foraging on land and they have salt glands so they can eat salty marine invertebrates like crabs.

 White ibis probe in aquatic vegetation
& catch mainly crayfish
Ibis catching crayfish 
 White ibis on lawn will probe
& catch mainly cockroaches
Ibis catching cockroaches 
 White ibis probe at seashore & catch mainly crabs Ibis catching crabs 
Despite all of these advantages over wood storks, white ibis nesting success has declined drastically since the Everglades was drained and reduced by 50%. From the 1930's through the 1940's there were four years with 4,000 to 10,000 nests, mainly in the southern Everglades National Park at the salt-fresh water estuary. The best years since had 2,000 to 3,000 nests with none in the ENP estuary. And the interval between great nesting years has increased from about 2 years to 5 years. The net decrease in total nesting has been 87%. During this same time the great egret nesting success has increased > 200%.


Ardea VISUAL FEEDERS
ARE THE MOST SUCCESSFUL NESTERS

As our largest visual feeders with a "crumb-picking" feeding mode, Ardea wading birds, the great egret and the great blue heron, are even more regularly successful nesters than white ibis. In the three water conservation areas including our Refuge, both nested successfully in 70 - 90 percent of years. In 2009, one of the best nesting years on record, great blue herons nested in 70% of the 44 rookeries and great egrets in 93% of the rookeries. By contrast the bill probe feeding white ibis nested in 20% of the rookeries and the bill gape feeding wood stork in only 11% of the rookeries.

The largest visual feeders, great egrets and great blue herons (Ardea), have more uniform nesting and feeding success than smaller visual feeding snowy egrets and little blue herons (Egretta). With long legs, Ardea species can feed in both deep and shallow water. They can eat big prey and so get maximal nutrition per time with their sit and watch foraging mode. And they can apparently feed at night if necessary.













Great blue heron with fish
Great egret with frog 
Great blue heron with a very big catfish   Great egret with a big frog

REVIEW QUESTIONS

a. Water quantity, distribution, and timing have to be "just right" for wood storks to nest successfully.

b. With an El Niño year in 2009-2010 we can expect highly successful nesting of wading birds including white ibis and wood storks.

c. Wood storks are "cream skimmer" feeders.

d. Wood storks are more generalized feeders than white ibis.