Friday, June 4, 2010

Roseate Spoonbill

Roseate Spoonbill

© Juan C Aguero (juanKa)
Photos 2009-2010


Nature Of Migration In The Species

Best described as dispersive after breeding, probably throughout entire range in response to local environmental conditions. Certain populations may also regularly migrate. Postbreeding dispersal often related to drying and flooding cycles, as well as sub-sequent food availability. In Costa Rica and Yucatán Peninsula, this species moves inland when coastal marshes and mangroves have dried out and rainy season has commenced (Stiles and Skutch 1989, Ornat-Lopez and Ramo 1992). In Florida, postbreeding dispersal of juveniles to interior marshes of mainland varies from year to year; few individuals were present during one year of deep flooding (1979; Robertson et al. 1983). In Louisiana, after breeding, individuals disperse inland in response to concentrated food from drawn-down crayfish and catfish ponds at farms (S. Cardiff pers. comm.). Age classes may have different distribution and seasonal patterns of movement, but not definitive (Robertson et al. 1983). Northward dispersal in U.S. can be extensive and irruptive in some years, consisting of mostly immature birds. See Distribution: the Americas, above.

Timing And Routes Of Migration

Presence of this species in U.S. breeding range throughout year makes it difficult to characterize movement. The small number of band recoveries and sightings of marked individuals beyond first year hinders analysis of age and movement (Robertson et al. 1983, Telfair and Swepston 1987, Powell and Bjork 1990). Initial dispersal is northward, at least by juveniles and other immatures after breeding season; this dispersal is followed by movement south for winter (see Demography and populations: range, below). Ju-veniles leave Florida Bay colonies in Mar and disperse northward; adults leave colonies in Mar or Apr and do not return until Oct or Nov for winter breeding; summer destination of adults largely undetermined (Allen 1942, Robertson et al. 1983, Bjork and Powell 1996). Likely includes parts of peninsular Florida where adults may renest (see Fig. 1); and Cuba (Allen 1942). Observed in transit between Florida and Cuba in both directions (A. Sprunt IV pers. comm.). In Texas, occupies breeding colonies until sometime in Aug; returns in late Feb, Mar, and Apr. Records indicate dispersal northward in late summer and fall along watercourses, including inland (Oberholser 1974). Band recoveries of adults and juveniles in Mexico during winter document presence of Texas birds along Gulf Coast from Tamaulipas south to Veracruz, but 1 bird was recovered in winter in Louisiana as well (11 recoveries in Mexico, 9 coastal, n = 910 banded, 1955–1983; Telfair and Swepston 1987). Withdrawal in winter is not complete; occa-sionally winter numbers remain similar to those in summer along coast (Allen 1942, Oberholser 1974, Rappole and Blacklock 1994).

Extent that nonbreeding and breeding populations in U.S. are augmented by migrants or recruitment from outside U.S. is poorly understood (reviewed by Robertson et al. 1983; see Demography and populations: population status, below). Summer population along southwestern coast of Florida probably is augmented by northward movement of immature birds originating in Cuba or nearby (Allen 1942, but see Robertson et al. 1983), since the large numbers (average 500) of immatures during spring and summer 1941 could not be accounted for by the small numbers of breeders in the state at the time. Similarly in Texas, adults and immatures originating in Mexico probably contribute to spring and summer nonbreeding population (Bent 1926, Allen 1942). Birds from Mexico apparently were important in rapid repopulation of Texas colonies following decimation in early 1900s. Distribution in U.S. suggests that gene flow may occur between eastern and western U.S. populations via Yucatán Peninsula into Cuba, then Florida, since overlap apparently is minimal in Alabama and Mississippi. See Distribution: the Americas, above.

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