Genetic diversity of cactus wrens is declining in southern California

Prickly wrens nest in prickly pear trees along the southern California coast. Credit: Alex Houston

The high mountains of the Transverse and Peninsular Ranges separate the cactus wrens along the southern California coast from the rest of the species found in the interior southwest deserts. While the birds are doing fairly well in arid parts of their stronghold like Nevada and New Mexico, habitat loss has hammered coastal cactus wren populations in southern California as the human population was increasing.

“The distribution and numbers of birds have really declined as this habitat has developed,” said Amy Vandergast, a genetic researcher at the San Diego Field Station of the US Geological Survey’s Western Ecological Research Center. “This is a situation where we could see declines in genetic diversity.”

Coastal populations are generally found from Ventura County to San Diego and across the international border into northern Baja California in Mexico.

These non-migratory brown mottled wrens, which are a bit larger than your typical songbird, nest in cacti like prickly pears. Cactus Wrens (Campylorhynchus brunneicapillus) are listed as California State Species of Concernand Vandergast and his colleagues wondered how genetically and physically connected birds were around San Diego.

As part of the San Diego Management and Monitoring Program, researchers tracked the genetic diversity of four populations of cactus wrens around the city.

Coastal wrens are separated from other populations by mountains. Credit: Alex Houston

For a study recently published in Conservation science and practice, Vandergast and his colleagues started in 2011, playing sounds of male birds to help them trap birds in mist nets. They also analyzed chicks from known nests. In the first batch they collected, the team extracted DNA from blood or small feather samples from 76 birds across four populations. They then returned in 2017 and used the same technique to extract DNA from 113 other birds from a more recent generation. They only included one bird from each nest or family group in the analysis in an effort to sample each population evenly.

By comparing the results over time, the team found that genetic diversity decreased significantly in two of the populations. Genetic information also revealed that there was a low amount of genetic movement between populations. Cactus wrens do not migrate – they rarely move more than a few miles outside their home range in their lifetime.

Taken together, these two pieces of evidence paint a grim picture of cactus wrens in the San Diego area. “If the populations remain this small, we can expect that within each of them the genetic diversity will continue to decline over time,” Vandergast said.

These declines may be exacerbated by prolonged drought in the region. If genetic diversity continues to suffer, Vandergast said wildlife managers may need to consider taking action, including swapping eggs from one population in the nest of another. However, this would only provide temporary solutions for some populations. To make long-term improvements in genetic diversity, managers need to consider actions such as habitat restoration to connect isolated populations, Vandergast said.

In the meantime, Vandergast said genetic monitoring will continue to be important for this population of wrens.

“The coastal cactus wren is a really interesting and unique songbird in Southern California,” she said. “And he may be in trouble. When populations lose their genetic diversity, they may lose their ability to adapt to changes in their habitat and environment.

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