Flying Free Once More: Nest Guarding in the Reintroduced California Condor

by Estelle Sandhaus, PhD

The critically endangered California condor remains a highly publicized symbol of wildlife conservation. From a global population of just 27 captive adults in 1987, ex situ breeding produced a sufficient number of individuals to commence reintroduction of young birds into the skies of southern California in 1992.

One key challenge to the recovery of condors was abysmally low wild population growth following the onset of breeding in the reintroduced Southern California population. This was due to low nestling survival rates. From 2001–2006, average nest success (excluding one intensively managed nest) did not exceed 6 percent annually—far lower than the 41–47 percent reported in the remnant population by Snyder and Snyder (1989) and insufficient for a viable population.

The data from this period provided preliminary insight into the factors related to nest failure (e.g., egg failure, West Nile virus, and anthropogenic trash ingestion by chicks). Yet, the lack of more systematic data collection at nests precluded a sufficient understanding of the mechanisms and timing of nest failure and what could be done to prevent it. In 2007, therefore, the Santa Barbara Zoo, in partnership with the US Fish and Wildlife Service and other California Condor Recovery Program partners, initiated a formalized nest guarding program (NGP).

The first goal of the NGP is to maintain a continued observational presence at nests, combined with a systematic schedule of routine nest entries. This allows detection of problems that are addressed through intervention, and precise identification of the causes of individual nest failures if they do occur. Interventions include a West Nile Virus vaccination program, swapping of infertile eggs with fertile eggs from the captive breeding program, and temporary removal of nestlings for surgery to remove trash from the gastrointesinal tract. The second goal of the NGP is to use systematic data collection to answer specific research questions about nest failure so that effective long-term management solutions are implemented. Questions include whether certain condor pairs bring more trash to nests than do others, and whether trash load at the nest varies across the season. (As curious, scavenging birds, condors are attracted to microtrash such as broken glass, bottle caps, and can tabs, which may be mistaken for nutrient-rich bone fragments and can be deadly if ingested.)

With generous assistance from AWI’s Christine Stevens Wildlife Award, the NGP has been successful thus far in increasing nest success, reaching a high of 64.2 percent during the 2007–2015 period. Importantly, newer condor pairs have demonstrated less of a propensity to bring trash to the nest. This is due, perhaps, to organized trash cleanups and/or the fact that food subsidies have declined and the free-flying condors are spending more times foraging in wild areas away from humans and their trash. We have also more closely determined the timing of trash delivery, identifying 60–90 days of age to be a critical stage in terms of potential trash ingestion and impaction. We have documented breeding and nesting events, including nestling predation by a black bear and eight consecutive years of nesting by a single condor pair, heretofore undescribed in the literature.

Recently, we entered an exciting phase of the NGP: The first wild-fledged chicks are attaining reproductive maturity, and we will soon be able to compare their behavior with that of their captive-bred parents. While condors remain critically endangered and are still threatened by anthropogenic factors (e.g., lead and trash ingestion, habitat loss) the species appears to be on the road to recovery.

Snyder, N. and H.A. Snyder. 1989. Biology and conservation of the California condor. Current Ornithology, 6:175-267.

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