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Breeding of the Salt Marsh Shrew

Richard F. Johnston, Robert L. Rudd
DOI: http://dx.doi.org/10.2307/1376305 157-163 First published online: 27 May 1957

Salt marsh shrews (Sorex vagrans halicoetes) are common inhabitants of some of the San Francisco Bay salt marshes in California. Their numbers in favorable marshes form perhaps 10 per cent of the mammalian fauna present, the rest of which is chiefly composed of Microtus californicus, Rattus norvegicus, Mus musculus, and Reithrodontomys raviventris, all of which are more abundant than shrews. There is no study yet reported on the biology of salt marsh shrews, and indeed little on the species S. vagrans throughout its range. Comparative material in this paper has been drawn mainly from Clothier (1955), Hamilton (1940), (1944), (1949), Jameson (1955), Blossom (1932), and Dehnel (1952); only the first of these deals with Sorex vagrans.

Field work was done from 1949 to 1955, although the major part of this time was spent on studies other than this one. Trapping was undertaken on all San Francisco Bay marshes inhabited by S. v. halicoetes; observations on nesting activity were carried out on San Pablo marsh, Richmond, Contra Costa County; a collection of owl pellets was also made at this locality. Observations on captive shrews were made at the Museum of Vertebrate Zoology (Rudd, 1953).

The environment.—The San Francisco Bay salt marshes are in an area of Mediterranean climate. Temperatures average perhaps 55° F. through the year, minima of 28° to 30° and maxima rarely of 90° to 100° occurring. Rainfall averages 18 to 25 inches a year and falls in the period from November to April. Thus, there is a long summer dry season. However, because of the summer high tides, dryness may reach critical levels for salt marsh shrews only in September and October.

The lower parts of salt marshes, 3 to 6 feet above sea level, are …

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