Wilson Bull., 109(4), 1997, pp. 720–731 720 LIFE HISTORY OF THE ENDANGERED CAPE SABLE SEASIDE SPARROW JULIE L. LOCKWOOD 1 , KATHERINE H. FENN 2 , JOHN L. CURNUTT 1 , DEBORAH ROSENTHAL 3 , KARLA L. BALENT 1 AND AUDREY L. MAYER 1 ABSTRACT.—Cape Sable Seaside Sparrows (Ammodramus maritimus mirabilis), breeding within eastern Everglades National Park, were philopatric and moved only short distances be- tween clutches. Incubation required 12 days, nestlings fledged at 9.2 days, and fledgling care ranged from eight to 20 days. The total nest cycle encompassed 34–44 days. Nestlings were fed spiders and insects, primarily Orthoptera, Lepidoptera, and Odonata. Diet varied between years and study sites. With the onset of summer rains, predation rate increased and nest success de- creased. Breeding activity diminished throughout June coinciding with rising water in nest areas. Our results indicated that the lack of breeding habitat and the onset of summer flooding limit the breeding potential of Cape Sable Seaside Sparrows. Received 19 October 1996 accepted 5 June 1997. In 1967, the Cape Sable Seaside Sparrow (Ammodramus maritimus mirabi- lis ), was classified as endangered (32 Federal Register 4001, 1967). Since 1992, its population has declined by more than 50% (Pimm et al. 1996). Its secretive habits and the general inaccessibility of its preferred habitat have long discouraged a comprehensive study of its breeding cycle and diet. Here we describe a two-year investigation of the biology of the species within east- ern Everglades National Park. In particular, we provide information on nestling diet, nest cycle duration, nest success and non-breeding activity. From these data, an accurate population model will be developed as an essential step in updating and implementing a recovery management scheme. Howell (1919) discovered the Cape Sable Seaside Sparrow in the sparse salt marsh prairies of Cape Sable. Ignorance of the south Florida interior led to the assumption that the sparrow’s entire range was in the Cape Sable area (Howell 1932). The September 1935 hurricane severely altered the landscape of Cape Sable causing the sparrow to disappear along with the prairies it in- habited (Pimm et al. 1996). Subsequent sightings of A. m. mirabilis were spo- radic and unreliable until L. A. Stimson’s search in 1955. Surprisingly, he identified several ‘colonies’ within freshwater marshes of the everglades (Stimson 1956). These sightings comprised the only known range of A. m. mirabilis until extensive surveys in 1981. 1 Dept. of Ecology and Evolutionary Biology, Univ. of Tennessee, Knoxville, Tennessee 37996. 2 South Florida Natural Resource Center, Everglades National Park, 40001 State Road 9336, Homestead, Flor- ida 33034. 3 340 SE Lilly Avenue, Corvallis, Oregon 97333.