Snail Kite
Breeding Potential Index (BPI)
Empirical basis and model assumptions:
Several types of habitat have been identified as suitable
habitat for snail kite reproduction.
These are, with their FGAP numbers:
Freshwater marsh (29,30), Typha (34), Spartina (35), Muhlenbergia (33,
39), Eleocharis (31), Openwater (0).
*
In the BPI other
types of habitat are excluded (the index for habitat types not listed is set to
zero).
Two additional factors affect the quality of habitat, as
they affect the availability of the snail kite's primary prey, the apple
snail. The first factor is the time
since the last drydown of the cell.
Apple snails are the primary prey of the snail kite. Since apple snails are aquatic and have a
limited capacity to survive dry conditions, drying events result in periodic
reductions in the availability of snail kite food resources. It takes more than two years for the
population of apple snails to recover following a drydown of the cell. A second factor is that an area of habitat
that has not dried down for too long a period may start to deteriorate as
suitable habitat.
*
In the BPI a drydown factor (dryfactor) is defined.
The dryfactor is related to the time since the last drying down of a
cell. Relative habitat quality on average
is about 50% of pre-drying conditions the year following the drying event, 85%
two years following and fully recovered by three years. A "wetfactor" is also
defined. It measures the possible
deterioration of the habitat due to being continuously inundated for long
periods. For this index model, cells which are inundated less than 80% or
greater than 98% of the time over a ten-year are considered unsuitable as snail
kite habitat; cells with inundation periods of 80-85% and 95-98% are considered
marginal; and cells with 85-95% inundation periods are considered suitable.
The snail kite may have more than one breeding cycle between
January 1 and July 31. The continuity
of a breeding cycle depend on the depth of water remaining continuously within
a certain range, such that apple snail in the water will be available to the
snail kites. The time to complete one
breeding cycle is estimated to be 110 days.
This is based on the time required for nest building (10 days), egg
laying (2-day intervals with incubation beginning with the 2nd egg), incubation
(27 days), the nestling period (30 days), and a post-fledgling period (45
days).
*
In the BPI, the index
is partially determined by the number of uninterrupted breeding cycles through
the breeding season, starting on January 1 through July 31. A minimum water depth of 20-cm at the time
of initiation is required for suitable breeding conditions. The continuation of a cycle depends on water
depths staying within a certain range. Water depths < 10-cm are considered
too shallow, so depth must remain above 10-cm for at least the time required to
successfully raise a brood (110 days).
Water that is too deep may also be unsuitable for breeding snail kites.
We defined an upper limit of suitable depths to be 115-cm.
Selected
references:
Beissinger, S.R. 1984.
Mate desertion and reproductive effort in the Snail Kite. Ph.D. Diss.
Univ. Michigan, Ann Arbor. 181
pp.
Beissinger, S.R. and N.F.R. Snyder. 1987. Mate desertion in the Snail Kite. Anim.
Behav. 35: 477-487.
Bennetts, R.E., M.W. Collopy, and S.R. Beissinger. 1988.
Nesting ecology of Snail Kites in Water Conservation Area 3A. Dept. Wildl. and Range Sci., Univ. Florida, Florida Coop. Fish and Wildl. Res. Unit, Tech. Rep. No.
31. Gainesville, Florida.
Bennetts, R.E., M.W. Collopy, and J. A. Rodgers, Jr. 1994.
The Snail Kite in the Florida Everglades: a food specialist in a changing environment. Pages 507-532 in S. M. Davis and J. C. Ogden
(eds.) Everglades: the ecosystem and its restoration. St. Lucie Press, Delray Beach, FL.
Bennetts, R.E. and W. M. Kitchens. 1997. The Demography and
Movements of Snail Kites in Florida.
US. Geological Survey/Biological Resources Division, Florida Cooperative
Fish and Wildlife Research Unit.
Technical Report No. 56, Gainesville, Florida.
Bennetts, R.E., W.M. Kitchens, and D.L. DeAngelis. 1998. Recovery of the Snail Kite in Florida:
Beyond a reductionist paradigm.
Transactions North American Wildlife and Natural Resources Conference
63: in press.
Snyder, N.F.R.,Beissinger, S.R., and R. Chandler. 1989. Reproduction and demography of the Florida
Everglade (Snail) Kite. Condor 91:
300-316.
Stieglitz, W.O., and R.L. Thompson. 1967. Status and life history of the Everglade
Kite in the United States. Special Sci.
Rept. Wildl. No. 109, U.S.D.I.,
Bur. Sports Fisheries and Wildl.,
Washington, D.C. 21 pp.
Sykes, P.W., Jr. 1987.
Snail Kite nesting ecology in Florida.
Florida Field Naturalist 15:
57-70.
Flow Chart for Construction of
Snail Kite Breeding Potential Index
The flow
chart shows the steps in computing an index value for a cell:
Variables
of index computation (top box):
Several types of habitat have been identified as suitable
habitat for snail kite reproduction and the FGAP types that are considered
suitable are listed here.
Cycle through days of
year to determine breeding conditions (middle):
The model cycles through the breeding season, starting on
January 1 and ending on July 31, to compute the number of possible breeding
cycles in a cell. A minimum water depth
of 20-cm at the time of initiation is required for suitable breeding conditions. The continuation of a cycle depends on water
depths staying within a certain range. Water depths < 10-cm are considered
too shallow, and thus depth must remain above 10-cm for at least the time
required to successfully raise a brood (110 days). Water that is too deep may also be unsuitable for breeding snail
kites. We defined an upper limit of suitable depths to be 115-cm. Each time a full cycle is completed, the
number of complete cycles for cell (x,y), NC(x,y), is incremented by 1.
Calculation
of total BPI (bottom):
The fraction of cycles that can be achieved, calculated
above, is multiplied by the smaller of two factors, the "wetfactor"
or the "dryfactor". The
dryfactor is related to the time since the last drydown of the cell, as shown
in the graph. The two years following a
drydown of a cell are characterized by 50% reduction in the index in the first
year and a 15% reduction in the second year. The wetfactor measures the
possible deterioration of the habitat due to being continuously inundated for
long periods. The wetfactor is equal to
1.0 only if a cell has been flooded for between 85% and 95% of the preceding 10
years, and is equal to zero if the percentage of time has been either < 80%
or > 98%. Linear interpolation is
used between 80% and 85% and 95% and 98% to obtain the wetfactor between those
values. The total BPI, termed
IndexMap(x,y), is the product of NC(x,y)/MaxNC and the wetfactor(x,y) and
dryfactor(x,y).
For more information, see Original Model Description