Nestling diets of coexisting salt marsh sparrows: Opportunism in a food-rich environment |
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| Authors: | William Post Jon S Greenlaw |
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| Institution: | 1. Charleston Museum, 360 Meeting Street, 29403, Charleston, South Carolina
2. Biology Department, Long Island University, 11748, Greenvale, New York
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| Abstract: | The saltmarsh sharp-tailed sparrow (Ammodramus caudacutus) and the morphologically similar seaside sparrow (Ammodramus maritimus) occur together in Atlantic coast salt marshes. Over 2 yr we examined food exploitation patterns to gain an understanding
of how these species coexist and to characterize the trophic conditions that allow the maintenance of different mating systems
within the same habitat. In the promiscuous saltmarsh sharp-tailed sparrow, only females feed the young, while the young of
the socially monogramous seaside sparrow are fed by both parents. To determine dietary patterns, we used a nonlethal method
(throat constriction) to obtain food samples from nestlings. Resource states were determined by sweep-netting the vegetation
and by taking cores from the marsh surface. Based on volume, the same 4 prey groups (flies, amphipods, grasshoppers, moths)
composed > 80% of each species diet. The main food of both sparrows was adult and juvenile (larval and pupal) soldier flies
(Stratiomyidae:Odontomyia microstomata). Measures of dietary overlap, based on resource states, indicated that nestling diets did not differ between species on
a seasonal basis or over 2-wk intervals. As the season progressed, each species tracked in parallel changes in resource abundance.
Diets did not diverge from what was expected if food was exploited randomly; diets frequently overlapped significantly more
than expected. Interspecific resource-use patterns were congruent over 2 yr. Based on the results of this study, and on an
evaluation of hypotheses based on life history information from previously published work, we conclude that food usually is
not limiting for these sparrows and that resource abundance has fostered behavioral and dietary opportunism. |
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