Distribution, Population Genetic Structure and Demography of Coastal Cactus Wrens in Southern California

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Dr. Barbara Kus studies the demography and genetic structure of remaining populations of the coastal Cactus Wren to help managers develop strategies to increase the stability of wren populations.

The coastal Cactus Wren (Campylorhynchus brunneicapillus) is a fragmentation-sensitive resident species in southern California requiring thickets of cholla or prickly pear cactus for nesting. Limited naturally by the patchy distribution of this habitat, Cactus Wren populations have become further fragmented in recent decades by urbanization, habitat degradation, and stochastic events such as wildfire. As a result, Cactus Wren populations have been diminished in size and distribution, and occur largely as islands in a matrix of generally unsuitable habitat.

Among the possible consequences of fragmentation on Cactus Wren viability is genetic isolation. This could limit the ability to adapt to changing environments as a result of loss of genetic variability. One key process by which genetic connectivity is achieved is juvenile dispersal, where young birds leave their natal territory and establish their own breeding territory elsewhere. This stage of the life history is poorly understood and requires more research to answer key questions.

A coastal Cactus Wren displays its singing power. (Public domain.)

In addition to isolation, steep declines in Cactus Wren populations have been detected recently in the southern part of San Diego County. This region has experienced multiple years of drought which may have affected wren abundance by reducing arthropod food resources, which could lower fecundity and/or survival. Improving habitat quality through management could increase food availability and enhance wren productivity and survival. Developing management strategies to increase the stability of wren populations in years with low rainfall is of particular importance if droughts become more frequent, intense and prolonged in the future, as predicted by climate change models.

Genetic Structure

Evaluating the degree of genetic connectivity among coastal Cactus Wren populations in southern California is a high priority need for effective management of populations occupying preserved sites within a fragmented landscape. The results of this research will provide information on Cactus Wren genetic variability and gene flow among populations that can be used to inform decisions regarding current and future corridor needs for maintaining viable Cactus Wren populations in coastal California.

Demography of Declining Populations

To identify factors responsible for population declines in southern San Diego County we are examining Cactus Wren productivity (number of young fledged per pair) and survival and their relationships to precipitation, habitat quality and arthropod food availability. The results of this research will help identify factors limiting productivity and survival and provide guidance on managing existing and restored  cactus scrub habitat to increase Cactus Wren population abundance and resilience.