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__Long- and Short-legged Wading
Bird Breeding Colonies__

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__Foraging Potential Index (FPI)__

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**Empirical basis and model assumptions:**

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The wading birds in a
wading bird breeding colony exploit an area of many square kilometers around
the colony site. A colony site, to be
successful, should be surrounded by sufficient foraging habitat.

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To construct the FPI,
all spatial cells within a 1.5 km radius (short-legged wading birds) and 3.0 km
radius of the colony site (cell or pixel) are considered to be in the primary
"foraging areas" and are evaluated. Only
cells that have sufficient surrounding cells that qualify as habitat (> 50%)
and less than 25% urban area are considered a potential colony sites. Others are excluded (index set to zero).

Wading birds can only
exploit fish from an area when water depths are within certain ranges.

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In the FPI
long-legged wading birds are assumed to require water levels in the range 5 to
35 cm and short-legged wading birds are assumed to require water levels in the
range 0 to 20 cm.

Wading bird breeding
can occur roughly between December and July and wading birds require a
continuous supply of available food for the entire period they are caring for
eggs and young. This requires that a
high enough fraction of their foraging area be in the correct depths, with
water depths going down, so that fish are concentrated.

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In the FPI the
breeding period for short-legged wading birds was chosen to be January 1 to May
15, while the breeding period for long-legged wading birds was chosen to be
from January 1 to July 15. The
foraging potential index for the region around a particular colony site or cell
is calculated to reflect that a sufficient fraction of the cells in the primary
foraging areas are in right depth conditions with decreasing water depth. In the index model "foraging
cycles" of 54 and 21 days are assumed for long-legged and short-legged
wading birds, respectively. The index
keeps track of the number of cycles of these lengths for which at least 20% of
the cells in the primary foraging area are in appropriate depth regimes with
decreasing depths. These "foraging
cycles" do not correspond to the actual breeding cycles, which are much
longer. However, they are assumed to be
reasonable surrogate measures of the goodness of conditions. If the mean suitable area surrounding a
colony decreased below 20% of the total area, the current wading bird breeding
cycle for that pixel was terminated and calculations for a new cycle not
initiated until the area mean increased
above 20%.
If a reversal of water depth takes place over a sufficiently large
fraction of the foraging area (> 80%), the cycle is terminated.

Wading bird breeding
success depends on the number of continuous days during the breeding season in
which breeding conditions are suitable, as well as the percentage of cells in
their foraging area that are habitat.

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The index
for a cell is computed based on the predicted number of breeding cycles
multiplied by a factor relating to the fraction of habitat cells in the
foraging area. The maximum possible
index value is 1.

**Selected references:**

Bancroft, G.T., A.M.
Strong, R.J. Sawicki, W. Hoffman, and S.D. Jewell. 1994. Relationships among
wading bird foraging patterns, colony locations, and hydrology in the
Everglades. In Everglades: The
Ecosystem and Its Restoration, S.M. Davis and J.C. Ogden (Eds.), St. Lucie Press, Delray Beach, Fla.,
chap. 25.

Cramer, P., K.M.
Portier and D.M. Fleming, D.M. 1997.
Systematic Reconnaissance Flights, Wading Bird Study, ENP. www.stat.ufl.edu/~arcs/enp/.

Fleming, D.M., W.F.
Wolff, and D.L. DeAngelis. 1994.
Importance of Landscape Heterogeneity to Wood Storks in Florida
Everglades. Environmental Management
18(5):743-757.

Frederick, P.C. and
G.V.N. Powell. 1994. Nutrient transport
by wading birds in the Everglades. In Everglades: The Ecosystem and Its
Restoration, S.M. Davis and J.C. Ogden
(Eds.), St. Lucie Press, Delray Beach,
Fla., chap. 23.

Ogden, J.C.
1994. A comparison of wading bird
nesting colony dynamics (1931-1946 and 1974-1989) as an indication of ecosystem
conditions in the southern Everglades.
In Everglades: The Ecosystem and Its Restoration, S.M. Davis and J.C.
Ogden (Eds.), St. Lucie Press, Delray
Beach, Fla., chap. 22.

__
Flow Chart for Construction of
Wading Bird Colony Foraging Potential Index
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The flow
chart shows the steps in computing an index value for a cell:

__Variables
of index computation (top box):__

To
evaluate the index for a cell, the conditions of all the cells with a **radius** of 1.5 km for short-legged
wading birds and 3.0 km for long-legged wading birds are considered. Those cells are considered to be the
"primary foraging area" of the wading birds. Only 500-m cells with the listed FGAP
habitat types are considered. **Habitat cutoff (%) **means that only
those pixels with 50% of their primary
foraging area being of those habitat types can be considered as sites for colonies. Similarly, **Urban cutoff (%)** means that only pixels with less than 25% urban
area can be sites for colonies. **Reversal
threshold (%) **means that if there is a 20% in the water depth of a cell
from one time step to the next, that cell is defined as being in reversal. If a fraction of 100% - **Foraging area
reversal (%) **of the cells in the primary foraging area of the colony
undergo water level reversal, then the current foraging cycle is ended. The primary criteria used in the index are (1) the
occurrence of water in a specific depth range for each wading bird type; that
is, between **LoWaterlim** and **HiWaterlim** (0-20 cm for short-legged
wading birds and 5-35cm for long-legged wading birds). The foraging cycles for the two wading bird
types are given by **Cycle length. **These respectively have foraging cycles
of 21 and 54 days.

__Cycle through days of
year to determine breeding conditions (middle):__

The model
tracks hydrologic conditions from **start
season date** until **end season date.** Starting at the beginning of the
reproductive season, the model checks the status of all 500-m cells within the
primary foraging radius of the colony (only those cells that qualify by being
of the right habitat type and having had hydroperiod > 150 days the
preceding season. It checks two things,
(1) whether a given cell is within the correct water depth range, and (2)
whether water depth is decreasing. If
conditions are satisfied over at least 20% of the cells in the primary foraging
area, then the current foraging cycle is continued. If these good foraging conditions hold for 21 or 54 days, for
small or large wading birds, respectively, then one complete foraging cycle is
recorded and a new one is started. If the mean suitable area surrounding a pixel decreases
below 20% of the total area, the current wading bird breeding cycle for that
pixel is terminated and calculations for a new cycle not initiated until the
area mean increases above 20%.

A
reversal is defined as a greater than 20% depth increase over the previous time
step. If a reversal of water depth takes place over a sufficiently large
fraction of the foraging area, the current foraging cycle is terminated. If more than 100% - **Foraging area reversal
(%) **is in reversal (currently, 100% - 20% = 80%), then the current foraging
cycle is terminated, and a new one begun when foraging conditions are good
again.

__Calculation
of total BPI (bottom):__

The index
for a cell is composed of two factors.
The first factor, **NC(x,y)/MaxNC**
is found by summing the number of predicted foraging cycles for the colony and
dividing by the maximum possible number of foraging cycles. The second factor, **FeedingRadiusFactor(x,y)**,
is the fraction of cells in the primary foraging area that are of
suitable habitat type.