Cape Sable Seaside Sparrow

Breeding Potential Index (BPI)



Empirical basis and model assumptions:


Cape Sable seaside sparrows prefer dry marl prairie habitat that is dominated by Muhlenbergia or sparse Cladium grass. The quality of the habitat improves as the percentage of these grass types in an area increases.  The quality of a cell also depends on there being a sufficiently long hydroperiod during the wet season (to produce sufficient insect prey). The sparrows do not nest in the vicinity of trees or other woody vegetation.


*         Therefore, in the BPI model only spatial cells with > 15% cover by Muhlenbergia or sparse Cladium are included.   Cells containing or adjacent to cells with woody vegetation are excluded.  Two habitat quality factors, (1) percent of Muhlenbergia or sparse Cladium and (2) length of hydroperiod in preceding year modify the index value.


Nesting can occur from January 1 to June 30.  Male defense of territory begins when water depths fall below approximately 5 cm, and a reproductive cycle can start at that point.


*         Therefore, the index model tracks conditions from January 1 to June 30.  When the water depth falls below 5 cm in a cell, the index model starts to add up the continuous days for which depth stays below that level.


One reproductive cycle lasts approximately 43 days (but 5 more days are needed if a male has to find a new territory).  A nest will be abandoned if water depths rise to 16 cm during the cycle.


*         The index tracks the number of potential reproductive cycles on a cell during a reproductive season.  As soon as the continuous number of days with water level below 16 cm reaches the time needed to complete a breeding cycle, the index value is incremented.  If a reproductive cycle is interrupted before completion, no addition is made to the BPI.


Up to three complete reproductive cycles are possible for the sparrow.


*         The total BPI consists of three factors; (1) the total number of cycles in a year, which is maximum for three, (2) a factor representing percent of Muhlenbergia or sparse Cladium in a cell, and (3) the length of hydroperiod in the preceding year.  The maximum value for the BPI is 1.0.



Selected references

Nott, M.P., O.L. Bass, Jr., D.M. Fleming, S.E. Killeffer, N. Fraley, L. Manne, J.L. Curnutt, T.M. Brooks, R. Powell and S.L. Pimm. 1997.  Water levels, rapid vegetational changes, and the endangered Cape Sable seaside sparrow.  Animal Conservation (in press).


Lockwood, J.L., K.H. Fenn, J.L. Curnutt, A. Mayer and D. Rosenthal. 1997. Natural history of the Cape Sable seaside sparrow. Wilson Bulletin (in press).


Flow Chart for Construction of
Cape Sable Seaside Sparrow Breeding Potential Index



The flow chart shows the steps in computing an index value for a cell:


Variables of index computation (top box):


init_depth  -   water depth below which the territorial defense by males can start.

interrupt_depth  -  water depth at which nest abandonment will occur.

n_exploredays  -  number of days a male needs to explore to find a territory.

n_mating days - number of days required before egg-laying occurs on a territory.

walk_age  -  number of days from egg laying before nestlings are safe from flooding.

fledge-age  - number of days from egg laying before nestlings are fledged and new breeding cycle can begin.


Only 500-m cells with < 15% cover by Muhlenbergia and/or sparse Cladium grass are suitable.



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


The model tracks hydrologic conditions from the start until the end of the breeding season (1 January to 30 June).  The CSSS males claim territories and start a nesting cycle only when water level has decreased below 5 cm (init-depth).  When water depth in a 500-m cell is less than 5 cm., 5 days (n_explordays) are assumed required for a male to find the cell, 5 days (n_matingdays ) for mating to occur (only in the first cycle), and 33 additional days (walk_age) for nest to produce walking young.  At any time before the completion of a cycle of mating and offspring reaching the walking stage (38 days), a rise of water to 16 cm (interrupt_depth) can cause desertion of the nest.  We assume breeding was unsuccessful and the nestlings die. One nesting cycle is reached when the 40 days  following mating (fledge_age) are reached.  After nest desertion, or when a brood has been brought to the fledgling stage, the male can start another nesting cycle in a cell, if the water level is less than 5 cm (init_depth).  Up to 3 cycles (MaxCycles) are possible in a breeding season.


Calculation of total BPI


IndexMap(x,y) represents the value of the index of a cell specified by the coordinates (x,y).  The first factor in the index for a cell is the ratio of the number of possible cycles on a given cell, n_cycles(x,y) in a given year to the theoretical maximum, MaxCycles.  This is multiplied by two factors:  a site_factor(x,y) (percent of Muhlenbergia or sparse Cladium) and a hydroperiod_factor(x,y).


The site_factor(x,y) is a weighting factor between 0 and 1 which is the maximum of:


(1)   a habitat factor based on % Muhlenbergia in the cell (percentage of 30-m pixels in the 500-m cell which are classified as Muhlenbergia)(if less than 15%, habitat factor = 0);


(2) a location factor based on documented nesting success from past monitoring. (A "1" is assigned to cells where birds are known to nest, and to the 8 surrounding  cells.


In other words, if a cell has < 15% Muhlenbergia and no birds have nested nearby, the index will be 0 for that cell (unsuitable habitat). If the cell has either proven nesting success or Muhlenbergia > 15%, it is considered suitable habitat.


The hydroperiod_factor(x,y) is a factor between 0 and 1 which is designed to reduce the index after long periods of dryness or inundation that would cause vegetation change to an unsuitable type. Without such a factor, values in the eastern and core sparrow areas would be uniformly better in drier scenarios, even though fire and shrub invasion following dry conditions would cause habitat changes unfavorable to sparrows. The hydroperiod_factor is not currently implemented. If the model is recalibrated with more recent monitoring data and revised, this factor will be implemented.

  For more information, see Original Model Description


Return to ATLSS SESI Model Documentation Page
Return to ATLSS Home Page
Return to ATLSS Initial CERP Update Home Page