Factors Influencing Nontarget Bird Occupancy of Restored Wetlands in California’s Central Valley

Abstract

Intensively managed restored wetlands and flooded croplands of California’s Central Valley support millions of wintering waterbirds. While the benefits to wintering waterfowl are well documented, the effect of intensive management on birds at other times of the year is less clear. Practices such as drainage, mowing, disking and burning may be a nuisance to these nontarget birds at best or life threatening at worst. Alternatively, irrigation over the summer may create habitat that might otherwise be lacking in the dry season. Our objective was to assess the influence of management, adjacent land use, and habitat characteristics on the richness, diversity, and occupancy of birds other than waterfowl in the spring and summer. We conducted 640 bird surveys on restored wetlands managed at varying levels in 2008 (4 April–30 July) and 2009 (19 April–16 July) and used likelihood-based modeling to evaluate occupancy and the relative importance of intensity of management and various environmental factors. Management was not the most important predictor of the richness, diversity, or occupancy of nontarget birds in the summer; rather, variables such as wetland size, vegetation composition, and landscape characteristics were more important for most bird guilds. Contrary to the commonly held view that restored wetlands in California’s Central Valley support only wintering waterfowl, they also support a diverse avifauna year round regardless of how they are managed. Bird occupancy and diversity in restored wetlands may be enhanced by creating and maintaining large, complex mosaics of vegetation. Each year California’s Central Valley hosts the majority of the Pacific Flyway’s wintering waterfowl (Gilmer et al. 1982). Restored wetlands provide critical food and habitat for winter visitors, and there is broad support for programs and initiatives geared toward increasing the extent of wetlands in winter (CVJV 2006). Far less emphasis is placed on the role restored wetlands play in supporting summer breeders, migrants, and year-round residents. Greater than 95% of all depressional wetlands and 98% of riparian wetlands in California were destroyed between 1800 and 1980 (Dahl 1990, CVJV 2006, Garone 2011, Duffy et al. 2016). The passage of the Swamplands Act in 1850 and subsequent revisions led to extensive alterations to natural hydrology through the creation of reservoirs and channelization of the Central Valley’s main rivers. Restoration has been intended to replace the thousands of acres of wetland lost to agriculture and human settlement. Most managed restored wetlands target wintering waterfowl and therefore receive most of their water in the fall and winter. Historically, however, much of the