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