Reproductive success of the threatened San Clemente Bell’s Sparrow (Artemisiospiza belli clementeae) in recovering habitats is similar to success in historical habitat

Abstract

The San Clemente Bell’s Sparrow (Artemisiospiza belli clementeae) is a federally threatened subspecies endemic to San Clemente Island, California. Previous research suggested dependence on boxthorn (Lycium californicum) as breeding habitat and nesting substrate; however, this conclusion was based on data collected when introduced feral ungulates had severely degraded the soil and vegetation cover. Since removal of the ungulates, native vegetation has gradually increased and the San Clemente Bell’s Sparrows have expanded into areas where habitat had been unsuitable. To explore how Bell’s Sparrows use these areas, we examined reproductive metrics associated with habitat covariates gathered at 214 nest sites used by Bell’s Sparrows from 2014 to 2016. We found that nest success in boxthorn habitat, previously considered an essential habitat for Bell’s Sparrow nesting, was similar to success in alternative habitat types. Our findings contradict previous conclusions that Bell’s Sparrows were boxthorn-dependent. We believe this previously documented relationship was likely due to the lack of available alternative nesting habitat following years of feral ungulate degradation, and Bell’s Sparrows now reproduce in multiple habitat types and throughout most of San Clemente Island. Furthermore, our findings illustrate the importance of long-term monitoring and corresponding adaptive management when monitoring species in changing and recovering landscapes.

Nest survival and cause-specific nest mortality in the San Clemente Bell’s Sparrow

“…Although previous research found SCI Bell's Sparrows nested almost exclusively in California boxthorn (Lycium californicum)-dominated habitat (Kaiser et al 2009, Hudgens et al 2011, during this study period the sparrows also occupied the recently recovered alternative shrub cover, including sagebrush (Artemisia californica and A. nesiotica). We expected higher nest success in sagebrush than other substrates, given Bell's Sparrows affinity for sagebrush in other systems (Rich 1980, Misenhelter andRotenberry 2000), and observed higher nest success in taller shrubs (Meiman et al 2020). We expected that early-season nests would have higher survival than lateseason nests, given that research in semi-desert species suggests resources may decline late in the breeding season (Ruth and Skagen 2018).…”

“…The maritime desert scrub habitat at low elevation on the western side consisted of California boxthorn, interspersed with forbs and cactuses, including prickly-pear (Opuntia littoralis and Opuntia oricola), coastal cholla (Cylindropuntia prolifera), and golden-spined cereus (Bergerocactus emoryi; S. T. Meiman, S. A. Munoz, E. E. Deleon, B. Sandstrom, S. Nefas, and A. S. Bridges, 2017, unpublished manuscript). Bell's Sparrows have historically nested in this boxthorn-associated community at high densities (Meiman et al 2020). Along the higher elevation western terraces and eastern slopes, various combinations of cactus and forbs occurred, with small patches of low shrubs, the most common of which were sagebrush, coyote brush (Baccharis pilularis), and island morning glory (Calystegia macrostegia amplissima; Meiman et al 2017, unpublished manuscript).…”

“…San Clemente Bell's Sparrows build nests in low shrubs or other sturdy vegetation, with 39 plant species identified as nesting substrates (Kaiser et al 2009, Meiman et al 2020). Females lay one to five eggs, and the average incubation period lasts 12 days.…”

“…Both parents feed nestlings invertebrates, and fledging occurs approximately 11 days after hatching (Kaiser et al 2009). The length of the nesting season is positively correlated with precipitation, with nesting seasons lasting up to 177 days (Meiman et al 2020), allowing the sparrows to successfully nest up to five times during a breeding season (J. T. Stahl, A. S. Docherty, A. S. Bridges, B. R. Hudgens, and D. K. Garcelon, 2010, unpublished manuscript). Bell's Sparrows are not known to reuse nests, and researchers have documented consecutive nests built in the same shrub as the first nest, or in shrubs up to 100 m away (S. E. Ehlers, L.S.…”

“…Native vegetation cover on SCI was largely destroyed by non-native herbivores, which were removed from SCI during the 1970s-1980s and ultimately eradicated by 1991 (Keegan et al 1994). Since then, recovering vegetation has expanded the breeding range and nesting substrates of Bell's Sparrows (Meiman et al 2020).…”