Diet of the white-collared seedeater Sporophila torqueola (Passeriformes: Emberizidae) in texas.
White-collared seedeaters were studied at two sites in Zapata County, Texas. Site 1 was located on the banks of the Rio Grande River within the city of San Ygnacio (27[degrees]02'N 99[degrees]26'W) in a black willow (Salix niger) dominated community, with an understory of barnyardgrass (Echinochloa crus-pavonis), Louisiana cupgrass (Eriochloa punctata), spreading panicum (Panicum diffusum), Bermudagrass (Cynodon dactylon) and Mexican sprangletop (Leptochloa uninervia).
Site 2, a marsh bordering a pond in Zapata County Park (26[degrees]54'N 099[degrees]16'W), was located within the city of Zapata. The habitat was characterized by Bermudagrass, buffelgrass (Cenchrus ciliaris), Guineagrass (Panicum maximum), Johnson grass (Sorghum halepense), southwestern bristlegrass (Setaria scheelei), dock (Rumex chrysocarpus) and cattail (Typha domingensis). Trees included sugar hackberry (Celtis laevigata), black willow, huisache (Acacia minuata) and guajillo (Acacia berlandieri). Plant identifications follow that of Hatch et al. (1990).
METHODS AND MATERIALS
Observations were made from April to August 1995 at Site 1 (Eitniear & Rueckle 1995) and August to October 1994, February 1996, April 1997 and April 2000 at Site 2. Observations began at either 0800 h or 1000 h and continued to about 1800 h or 1900 h. Five birds were captured in mist-nets set at the site. Captured birds were leg banded and placed in a holding cage until a fecal sample was caught on blotting paper placed at the bottom of a small field cage. It was assumed these bird's fecal contents, although biased by a digestive differential of certain foods, provided a representative sample of recently consumed foods. The white uric acid covering was removed by flushing the sample with water. The remaining fecal mass was stored in 70% ethanol. Food items were identified by comparison to a reference collection of seeds and leaves from all plants at the study sites (Smith 1970; Servat 1993). Observations of foraging birds were conducted using 10 by 50 binoculars. Foraging observations were documented in a field notebook and a botanical specimen, from plants that contained seeds fed on, collected. Plant specimens were later identified by Robert Lonard (UT-Pan American). On occasion seeds were obtained from the mouths of captured birds. No effort, however, was made to flush crops.
RESULTS AND DISCUSSION
Items in the diet of the species are summarized in Table 1. The largest foraging group of seedeaters observed consisted of approximately 10 birds feeding on barnyardgrass and Louisiana cupgrass at Site 1. The birds fed throughout the day, frequently retreating to nearby black willows. Females were observed feeding Louisiana cupgrass seeds to recently fledged young at this location (Eitniear & Rueckle 1995). Fecal samples (five samples from five different birds) contained only barnyard and Louisiana cupgrass seeds, thus supporting the theory that grasses were the principle food resource consumed at this time. Green Louisiana cupgrass seeds in the milky stage of development were collected from the mouth and outer portions of the mandible of a female caught in a mist net. Plant succession altered this site significantly during the study. Black willow displaced barnyardgrass along the riverbank, and plains bristlegrass, buffelgrass, Guineagrass and blue panicum became established in open areas.
Seedeaters at Site 2 were observed feeding on southwestern bristlegrass, barnyardgrass and Louisiana cupgrass. Bermudagrass, Guineagrass, Johnsongrass and buffelgrass also were abundant, and contained ripe seeds, but not observed to be utilized as a food resource. Although grass seeds dominated observations of white-collared seedeaters diet, at 1200 h on 25 February 1996 at Site 1, a male foraged on huisache blossoms in a tree near the pond. For 30 minutes it was observed consuming the orange globose clusters of stamens. Subsequent to this observation, seedeaters had been observed feeding on the floral parts of willow (Table 1).
Bill morphology of the genus Sporophila favors seed eating (Cody 1985). Observations made during this study, although somewhat limited, support this concept. The greater proportion of barnyardgrass in the diet of the white-collared seedeater may reflect the greater abundance of this species over cupgrass and southwestern bristlegrass at Site 2 (Eitniear 1997b). Despite barnyardgrass growing abundantly on the opposite side of the pond at Site 2, seedeaters were never observed feeding on it; perhaps because no cover existed nearby.
Observations of feeding on the floral parts of willow and huisache in addition to records of its feeding on berries in Costa Rica (Stiles & Skutch 1989) and the pulp of Stemmadenia donnell-smithii in Mexico (McMiarmid et al. 1977) indicates greater plasticity in diet than previous authors have indicated (Cody 1985; Rubenstein et al. 1977). More research is needed to determine dietary shifts in this species in relation to changing seasons, variations in precipitation levels and landscapes. Such research may indicate if the decline of this species from a formerly robust widespread species in south Texas to the current patchily distributed remnant population is principally the result of the use of agrochemicals, habitat loss or some other factors (Eitniear & Rueckle 1996; Woodin et al. 1999).
Table 1. Parts of 12 plants consumed by Sporophila torqueola sharpei in Zapata, Zapata County, Texas, 1995-2000. Plant Species Part Eriochloa cruz-pavornis (seeds) Panicum maximum* (seeds) Echinochloa punctata (seeds) Panicum diffusum (seeds) Dichanthium annulafusum* (seeds) Panicum antidotale* (seeds) Cenchrus cilaris* (seeds) Setaria leucopila (seeds) Setaria scheelei (seeds) Acacia minuata (floral parts) Salix nigra (floral parts) Salix exigua (floral parts) *Non-native species
I wish to thank the numerous field assistants that participated in this study, especially Tom Rueckle. Robin Restall (Phelps Collection: Venezuela), Dr. John T. Baccus (Texas State University), Dr. Robert Lonard (University of Texas-Pan American), Dr. Keith Arnold (Texas A & M University, College Station), Dr. Timothy Brush (Uni-versity of Texas-Pan American) and Dr. Kent Rylander (Texas Tech University, Junction) and two anonymous reviewers contributed valuable suggestions to the study and/or manuscript. All birds were captured under permits from the Texas Parks and Wildlife and the National Biological Survey.
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Jack C. Eitniear
Center for the Study of Tropical Birds, Inc. 218 Conway Drive San Antonio, Texas 78209-1716
JCE at: JCE@cstbinc.org
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|Title Annotation:||General Notes|
|Author:||Eitniear, Jack C.|
|Publication:||The Texas Journal of Science|
|Date:||Feb 1, 2004|
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