July 2010
GOLDILOCKS CONDITIONS
FOR WOOD STORKS
Wood stork feeds
half-grown chick
Goldilocks, 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.
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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.
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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)
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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 |
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| 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.
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Group of
wood
storks foraging with open bills |
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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.
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Two fledgling wood storks
(fuzzy heads) learning to feed next to two adults
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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 |
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White ibis
on lawn will probe
& catch mainly cockroaches |
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| White
ibis probe at seashore & catch mainly crabs |
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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.
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| Great blue heron with a very big catfish |
Great egret with a big frog |
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