Avian Responses

Completed in 2009
            Breeding Bird Habitat Associations  Avian area-search surveys were conducted at 30 Tamarisk-removal sites and 30 control (mature tamarisk) sites to determine avian community responses to Tamarisk control treatments, along with avian point count surveys at 120 locations on the Virgin River from Littlefield, AZ to the Mormon Mesa just upstream of Lake Mead. These surveys, in conjunction with vegetation surveys at each study site provide comprehensive baseline (pre-beetle) habitat associations for all species breeding in the riparian habitat on the Virgin River, including species covered under the Clark County MSHCP, each of which were detected during surveys.  
            Avian Nesting Success  Over 150 nests of 19 riparian bird species were monitored for ten nest searching plots (five in Tamarisk and five in Tamarisk-Native mixed habitats).  These data, again in conjunction with vegetation surveys at nest sites, provide important baseline information on reproductive success of birds breeding in different habitats and are used to identify key habitat types for conservation or creation for critical bird species in the Virgin River system.  Tamarisk defoliation byDiorhabda may influence nesting success by increasing nest visibility to predators and/or exposure to the elements (e.g., sunlight, desiccation), or beetles may provide an abundant invertebrate food supply for birds and thereby increase nestling growth rates and nesting success.  In anticipation of beetles arriving at our survey sites we measured the temperature, humidity, vegetation density and sunlight penetration at each nest site. We also filmed nests to determine the amount of time parents spend regulating nest microclimate (e.g., incubating eggs, shading their offspring from sunlight) and the rates at which they fed their young.  These data will be used to determine whether changes in nesting success after beetle establishment can be linked to changes in vegetative cover at nest sites and or changes in food supply.
            Migratory Bird Habitat Associations  In October we used mist-nets to survey habitat of neotropical migrants and birds that overwinter on the Virgin River during the fall migration period.  These surveys were conducted near our point-count stations to provide baseline (pre-beetle) data on the habitat associations and densities of various migrant birds and will allow us to determine if migrants avoid patches that have been defoliated by beetles and/or prefer to forage in places where beetles are present in the fall because of the rich invertebrate food supply they afford.

Completed in 2010
           The number of avian point count surveys expanded to include areas targeted for restoration. During the 2010 breeding season (April – June) Diorhabda colonized the upper survey locations but had not yet spread throughout the drainage.This dichotomy provided both critical pre-beetle ‘control’ data from survey sites not yet colonized simultaneously matched with those areas where beetles emerged and dispersed during spring and early summer.  Nest monitoring continued and where beetles were present we collected samples of food delivered to the nest to determine the extent to which parents used beetles as a food resource for their offspring.  We also weighed nestlings at sites where beetles were present and absent to determine if beetles increased nestling growth and development. 

We implement a summer banding program for several target species (including the Arizona Bell’s Vireo) so we could track individual reproductive success throughout the breeding season (and the lifetime) of individuals in different habitats. Feces from adults captured in nets provided an assessment of which bird species use beetles as a food resource and to what extent.  By 2011 we expect beetles to be present in all of our survey sites and will collect comprehensive  beetle-present data on habitat associations, nesting success and habitat use by neotropical migrants and overwintering birds.