Mottled Ducks are the only dabbling ducks that both nest and winter in
large numbers along the Gulf Coast (Stutzenbaker 1988).
They also are the only non-migratory dabbling ducks
in the conterminous U.S. (Bellrose 1980).
Two populations of Mottled Ducks occur in North
America. One
population is a resident of peninsular Florida and the other is a
resident of the Gulf Coast from Alabama westward to Veracruz, Mexico
(Moorman & Gray 1994).
Mottled
Ducks were considered to be conspecific with American Black Ducks (Anas
rubripes), which they closely resemble, until the 1870s (Bent 1923). Like American Black Ducks,
male and female Mottled Ducks have similar plumage and are best
differentiated by bill coloration (Bellrose 1980). DISTRIBUTION: Mottled Ducks are common
in the extensive marshes located between Galveston Bay and Sabine Lake,
and locally common to uncommon along the remainder of the Texas coast
(Oberholser 1974, Stutzenbaker 1988).
They nest in all coastal counties, in most
first-tier inland counties, and in the coastal prairies (Oberholser
1974, Hobaugh et al. 1989). Notably
different than range maps in Oberholser (1974) or Stutzenbaker (1988),
TBBAP data suggest Mottled Ducks breed several counties inland along
the Rio Grande, with a brood sighting near the city of Zapata
(26099-H2) being the most westward confirmed record.
TBBAP data also suggest possible breeding in
Frio (28099-H1), La Salle (28099-C2), and Dimmit Counties (28099-E4). Probable
breeding accounts reported by TBBAP in northeast Texas are in an area
where attempts were made to propagate Mottled Ducks, and where past
sightings have been documented (Oberholser 1974, Stutzenbaker 1988). In one northeast Texas
quadrangle (33095-A1), Mottled Ducks were recorded for two consecutive
years during TBBAP surveys. SEASONAL
OCCURRENCE: Most
Mottled Ducks nest between February and July (Grand 1992, Stutzenbaker
1988), with the average (or peak) date of nest initiations occurring in
March, April, or May (Baker 1983, Grand 1992, Stieglitz &
Wilson 1968). Year
to year variation in timing of peak nest initiations can exceed 60 days
(Grand 1992). Low
water levels in late winter and spring are correlated with late nest
initiation peaks (Grand 1992). Extreme
dates reported for nesting activities include an early February nest in
Chambers County and a Christmas Eve brood sighting in Jefferson County
(Stutzenbaker 1988). BREEDING HABITAT: In the coastal regions of Texas and Louisiana Mottled Ducks nest primarily in dense stands of cordgrass (Spartina spp.) (Baker 1983, Stutzenbaker 1988). Other grasses and bush-forming plants also are utilized as nesting cover, but to a lesser extent (Engeling 1950, Stutzenbaker 1988). Mottled Duck nests are typically "built up" several centimeters from the ground with plant material or suspended above ground in dense stands of cordgrass (Engeling 1950, Rorabaugh & Zwank 1983). Over-water nests are extremely rare; in fact, Stutzenbaker (1988) documented only one. On 18 June 1991 a floating nest was found during TBBAP surveys; it was built on dead |
vegetation, water hyacinth (Eichhornia crassipes)
and debris lodged at the intake to a pump station (26097-A7) located on
the Rio Grande (W. MacWhorter, pers. comm.). Mottled
Ducks lay 8 to 12 eggs, which are covered with down and/or grass litter
when females take incubation breaks (Moorman & Gray 1994). Eggs range in color from
pale white to dull olive, but a single clutch will contain only eggs of
one color (Moorman & Gray 1994, Johnson et al. 1996). Eggs are incubated
approximately 26 days, and they hatch synchronously (Engeling 1950,
Stutzenbaker 1988). During
the time that eggs are hatching, some females will perform distraction
displays in attempts to lead predators away from their nests (W. P.
Johnson, pers. obs.). Mottled
Duck nests are typically dispersed, as the maximum density recorded in
Texas is only 1 nest/1.4 hectares (1 nest/3.5 acres) (Stutzenbaker
1988). Higher nest
densities have been documented only on dredge-spoil islands in Florida
and Louisiana (Stieglitz & Wilson 1968, Holbrook 1997). Nest parasitism (one
female laying eggs in another female's nest) has only been reported for
Mottled Ducks nesting on islands, and was probably the result of high
nest densities (Johnson et al. 1996). STATUS: Annual surveys of breeding
Mottled Ducks are conducted on several national wildlife refuges that
are located on the Texas coast. Densities
of pairs fluctuate widely from year to year, with low years partly
attributable to drought conditions (Neaville 1996).
Similar conclusions regarding the impact of drought
have been drawn in Florida (Moorman & Gray 1994). Analysis of Christmas Bird
Count data suggest Mottled Duck populations are not declining (McKenzie
et al. 1988). The
most serious threat facing Mottled Ducks is degradation and loss of
habitat (Stutzenbaker 1988). In
Texas, factors contributing to loss of habitat include agriculture,
urbanization, drainage, marsh subsidence, saltwater intrusion, and the
spread of introduced species (Stutzenbaker 1988, Morton & Paine
1990). Saltwater
encroachment into wetlands that range from fresh to moderately brackish
probably affects growth and survival of ducklings (Moorman et al. 1991). Encroachment of Chinese
tallow (Sapium sebiferum), an exotic tree, into
nesting habitat probably leads to abandonment of nesting areas
(Stutzenbaker 1988). Though
it is not yet a serious problem in Texas, Mottled Duck hybridization
with feral Mallards (Anas platyrhynchos) has the
potential to increase with continued human population growth along the
coast (Moorman & Gray 1994). Text by William P. Johnson (ca. 1998) |
