The 2 subspecies of swamp sparrows found in Maryland and Delaware have different strategies for raising young. The inland subspecies lives in the cool mountain bogs of Maryland, only a few hundred miles from its coastal cousin found in the salt marshes along the Delaware Bay.
The coastal birds tend to lay fewer eggs in a nest than their mountain relatives, but often re-nest several times during the breeding season. Recent research suggests 2 likely reasons for this disparity.
First, the coastal plain birds' nests suffer from more predation. Second, since the temperatures are higher along the coast, fewer eggs hatch. So for the coastal plain swamp sparrow, the adage "Don’t put all your eggs in one basket" makes perfect sense.
This article summarizes the information in this publication:
Olsen, B., Felch, J., Greenberg, R.S. and Walters, J. 2008. Causes of reduced clutch size in a tidal marsh endemic. Oecologia, 158(3): 421-435.
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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