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Causes of reduced clutch size in a tidal marsh endemic


We tested three hypotheses of clutch size variation in two subspecies of the swamp sparrow (Melospiza georgiana georgiana and M. g. nigrescens). Swamp sparrows follow the pattern of other estuarine endemics, where clutch size is smaller among tidal salt marsh populations (M. g. nigrescens) than their closest inland relatives (M. g. georgiana). Our results support predation risk and temperature, but not adult survival, as explanations of this pattern in swamp sparrows. Coastal nests were twice as likely to fail as inland nests, and parental activity around the nest site was positively related to clutch size at both sites. When brood size was controlled for, coastal adults visited nests less often and females vocalized less frequently during visits than inland birds, which may decrease nest detectability to predators. Coastal parents waited longer than inland birds to feed offspring in the presence of a model nest predator, but there was no difference in their response to models of predators of adults, as would be expected if coastal birds possessed increased longevity. Additionally, coastal females laid more eggs than inland females over a single season, following a within-season bet-hedging strategy rather than reducing within-season investment. Coastal territories experienced ambient air temperatures above the physiological zero of egg development more often, and higher temperatures during laying correlated with smaller clutches and increased egg inviability among coastal birds. Similar effects were not seen among inland nests, where laying temperatures were generally below physiological zero. Both subspecies showed an increase in hatching asynchrony and a decrease in apparent incubation length under high temperatures. Coastal individuals, however, showed less hatching asynchrony overall despite higher temperatures. Both air temperatures during laying and predation risk could potentially explain reduced clutch size in not only coastal plain swamp sparrows, but also other tidal marsh endemics.

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The field research was funded by grants from the Delaware Division of Fish and Wildlife, the Maryland Ornithological Society, the Washington Biologists’ Field Club, the Washington Group of the Explorer’s Club, the Eastern Bird Banding Association, the Smithsonian Institution’s Abbot Fund, Virginia Tech’s Graduate Research Development Program, the Bailey Fund at Virginia Tech, and both Smithsonian Institution Graduate and Pre-doctoral Fellowships. Much thanks to Kevin Kalasz, Jennifer Wang, Meghan Powell, Kari Murabito, Jaan Kolts, Karen Callaway, Audrey Wessel, Joel Adamson, Benjamin Augustine, Benjamin Beas, Kate Heyden, and Irene Liu for help in the field. All research was compliant with current laws of the USA. Bird handling was conducted under US federal banding permit number 22665 and approved by the Conservation and Research Center Institutional Animal Care and Use Committee of the Smithsonian National Zoological Park under proposal #04–10. Comments from Craig Osenberg, Blair Wolf, and an anonymous reviewer greatly improved this manuscript.

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Correspondence to Brian J. Olsen.

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Communicated by Blair Wolf.

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Olsen, B.J., Felch, J.M., Greenberg, R. et al. Causes of reduced clutch size in a tidal marsh endemic. Oecologia 158, 421 (2008).

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  • Egg viability
  • Predation
  • Survival
  • Swamp sparrow
  • Temperature