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.
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.
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