5. Bird Populations

Bird Populations as Indicators of Ecosystem Health


Riparian habitat in California is one of the most productive and valuable habitats for all forms of wildlife. Yet this is also one of the most threatened habitats, with only about 5% of the state’s original riparian habitat remaining. In addition to habitat loss, California’s remaining riparian and floodplain ecosystems have been greatly altered and impaired since the mid-19th century. Historically, winter and springtime flooding provided for extensive flooded plains that were used by native fishes. Such flooding is key to the maintenance of riparian forests, once the predominant floodplain vegetation in the Sacramento Valley. Currently on most rivers, however, the natural hydrologic regime has been altered by dams and levees that alter the timing and magnitude of flows. Changes in hydrological regime and loss of habitat have had severe consequences for birds that depend on riparian habitat.

The Cosumnes River Preserve and adjoining habitat provide an opportunity to study the ecological processes necessary to restore and maintain riparian habitat through semi-passive means: specifically, levee breaching and levee setbacks that will allow natural processes to proceed. Our study focuses on evaluating the condition of the ecosystem, reflected in both recently restored habitat (through semi-passive, natural processes and through planting) and in the remaining, remnant riparian forest and scrub, using birds as a study system. Our objectives with regard to bird studies are four-fold:

  1. to evaluate current conditions of bird communities and determine whether the current habitat (remnant and restored) is able to support stable or growing populations of riparian birds
  2. to gain insight into ecological processes (biotic and abiotic) that will maintain stable or growing populations as well as a diverse bird community
  3. to develop metrics of restoration success that can inform management practice
  4. to provide the foundation for a long-term monitoring program for riparian-dependent birds and wildlife.

In 1995, in cooperation with The Nature Conservancy (TNC), PRBO began evaluating and monitoring the riparian bird communities within the Cosumnes River Preserve. PRBO implemented a multitiered integrated monitoring program following nationally standardized protocols (Ralph et al. 1993). To assess the condition of the songbird community we collected information on habitat usage, species diversity and demographic parameters (e.g., reproductive success, Howell et al. 2006). Eleven years of data compilation on bird populations have been completed as of 2005.

In 2002, PRBO began to work with partners at UC Davis and UC Berkeley in the CALFED-funded study, The influence of flood regimes, vegetative and geomorphic structures on the links between aquatic and terrestrial systems: Applications to CALFED restoration and watershed monitoring strategies. With funding from the California Bay-Delta Authority, PRBO studied riparian bird populations during 2002-2005, with the aim of evaluating conditions of bird populations, improving our ability to evaluate restoration success, and, specifically elucidating the linkages between the terrestrial bird species and the aquatic ecosystem, which is characterized by intermittent flooding during winter and spring.

Operational Hypotheses and Objectives

In this study, we evaluate the following general hypotheses:

  • Bird population and community characteristics vary in space, at least partly due to restoration state (as reflected in age or type of restoration).
  • Bird population characteristics vary in time (over the duration of the 11-year study), partly due to hydrological conditions varying over time, but also due to other factors, including climate and weather variables.
  • Observed population parameters will reflect an interaction of space and time mediated by restoration state.

We also evaluate several more specific hypotheses:

  • Remnant riparian habitat is not of sufficient quality (due to habitat degradation, habitat fragmentation, incursion of non-native flora and fauna, and other influences) in order to maintain stable populations of riparian birds.
  • Restored habitat provides habitat that improves the condition of riparian bird populations, as reflected in population trends.
  • Population trends and demographic rates will differ among sites due to restoration state.
  • Time since restoration was initiated influences characteristics of bird populations and/or the avian community.
  • Populations of riparian birds respond, directly or indirectly, to hydrological regimes (such as flooding) and/or weather variables (such as rainfall).
  • The response to hydrological state reflects the food-web mediated linkage between aquatic and terrestrial ecosystems.

To evaluate these hypotheses, we analyzed abundance data at 12 study sites over an 11-year period for 22 bird species that use riparian habitat for breeding and analyzed reproductive success for one species, the Song Sparrow, at six study sites over the same 11-year period.

Restoration, Research and Management Recommendations

We provide restoration and management recommendations based on our results from 11 years of monitoring on the Cosumnes River as well as from other PRBO riparian studies from the Central Valley. Our results support many of the recommendations provided by the California Partners in Flight Riparian Bird Conservation Plan (RHJV 2004) where more detailed recommendations for are available (www.prbo.org/calpif/plans.html).

Increase floodplain connectivity and maximize winter flooding

Several of the Cosumnes focal species (Common Yellowthroat, Song Sparrow and Blue Grosbeak) prefer to nest in early successional habitat with dense understory cover. Early successional riparian habitat is dependent on floodplain connectivity and seasonal flooding which includes scouring, soil deposition and point bar formation. Results from the Cosumnes study show a positive correlation between Tree Swallow abundance during the breeding season and the number of winter flood days. In addition, Song Sparrow nest success in restored areas was also positively correlated with the number of winter flood days.

Manage for a mosaic of riparian habitat in different seral stages.

The importance of early successional riparian habitat cannot be overstated. The goals of conservation actions are often to re-create mature gallery forest. However, many bird species (Lazuli Bunting, Song Sparrow, Common Yellowthroat, Blue Grosbeak, Least Bell’s Vireo and others) either occur in low numbers or are absent in mature gallery forests. These species depend on early successional riparian habitat and are negatively correlated with characteristics associated with mature riparian (e.g., high canopy cover). We recommend managing for a mosaic of early, mid, and late successional habitat to benefit the full complement of riparian bird species.

Increase tree species richness

Planting or managing for high tree species diversity, especially large trees, will benefit many different bird species. Results from another songbird study in the San Joaquin Valley (Wood 2005) show that Bushtit and Western Scrub-Jay are positively correlated with tree species richness and studies from other sites (Nur et al. 2005) show a similar relationship with tree species richness and/or tree size (exemplified by House Wren and Tree Swallow, the latter a Neotropical migrant).

Increase understory plant volume and diversity

We recommend managing for understory growth particularly of herbaceous plants. This has been shown to be important to riparian birds in other studies (Holmes et al. 1999, DiGaudio 2001, Wood 2005, Nur et al. 2005). Mowing and spraying for weed control (e.g., Himalayan blackberry, Rubus discolor) at a site will necessarily reduce understory volume. The decision to mow should be carefully considered as a compromise between the need for weed control to promote a well-developed native understory over the long-term and the need to provide vegetative cover for ground-nesting birds during the important early successional stage of a restoration area.

Nest predator studies

We recommend studies of nest predators to better understand this threat and how it may be mediated by flooding and weather variables. Such studies will allow specific management actions to be formulated and evaluated that can mitigate predation rates. Recommended studies include continued nest monitoring using cameras to identify nest predators and quantify predation rates and Brown-headed cowbird activity at focal species’ nests. We also recommend analysis of nest survival and predation rates for all study species, especially for species that are currently declining in mature habitat. In addition, studies are needed to characterize predation risk (and the timing of predation) in relation to vegetation at or near the nest, as well as proximity of the nest to habitat edges (upland and river). A preliminary nest camera study at the Cosumnes River Preserve identified Brown-headed Cowbirds and black rats (Rattus rattus) as important nest predators (J. Hammond In prep.). Of 19 Song Sparrow nests monitored using cameras, 10 were depredated (6 by black rat [R. rattus] and 4 by Brownheaded Cowbird).