Living in southern California certainly has its ups and downs. We have beautiful weather (most of the year), gorgeous scenery, and the unique ability to experience a variety of ecosystems all within a day’s drive. We also have way too many people leading to gridlocked traffic, a scarcity of affordable housing, and, did I mention the traffic? And while it may cause us a little bit of heartburn from time to time to live here, it doesn’t compare to what our native wildlife has to adapt to and endure in order to survive in the only place they call home.
San Diego County is fortunate to be able to boast of many qualities, but the one that biologists are most proud to claim is that it is the most biologically diverse county in the continental United States. With that claim to fame also comes a hefty price, as the natural resources in San Diego County are also the most threatened in the country. San Diego County is host to approximately 200 plants and animals that are in decline and may be in danger of extinction—more than any other county in the nation. This alarming fact shines a light on a much bigger issue, though, that the status of these imperiled species is a reflection of the health of our ecosystems – ecosystems that are clearly in jeopardy. Just like the proverbial canary in the coal mine, our own coastal California gnatcatcher (Polioptila californica californica) is a prime indicator of the health of the coastal sage scrub vegetation community that once carpeted the landscape of coastal southern California. Development has replaced over 90% of southern California’s coastal sage scrub, and our little ‘canary’ must attempt to successfully breed in the remnant patches of habitat surrounded by freeways, housing developments, and strip malls. But this development, when mitigated properly, can also result in the protection of habitat, through the purchase of land to be set aside for conservation for species that will be impacted by the proposed development. These mitigation sites often undergo restoration to enhance the quality of the habitat, which makes it more attractive to species like the coastal California gnatcatcher and may increase the success rate of breeding pairs. This is the story of two of these gnatcatcher pairs…
In 2017, Balk Biological began conducting focused coastal California gnatcatcher surveys within a mitigation site that had been actively restored and was being continuously monitored and maintained. By all accounts, this 240-acre mitigation site is composed of some of the best coastal sage scrub this biologist has ever seen – lush and diverse vegetation with very little non-native and invasive species present in a large, contiguous swath of habitat that is off-limits to the general public. What more could a gnatcatcher ask for? So, the surveys began, with the expectation that there could be between 10 and 90 pairs…well, maybe not 90 pairs, but the gnatcatcher’s average territory size ranges from one to nine hectares, and the mitigation site is over 97 hectares in size (one hectare is equal to 2.47 acres). If you remove the chamise chaparral and the small patches of coast live oak woodland, non-native grassland, and western sycamores, it would not be out of the realm of possibility to have at least 10 pairs on the site. But the reality did not live up to the possibility, and only two pairs of coastal California gnatcatcher were detected on-site throughout the three rounds of surveys. Two pairs in over 200 acres. While it was disappointing to those rooting for this species that only two pairs were detected on-site, with just those two pairs to keep tabs on, the surveying biologist could focus more intently on trying to determine each pair’s breeding success. Due to the size of the survey area, each survey round took five days over the course of about two weeks to complete, which allowed the biologist a great opportunity to observe the nesting behaviors of these pairs, who will be referred to as Pair A and Pair B.
Pair A was observed to make at least four unsuccessful nesting attempts over the course of the 2017 surveys. The depredated nests were not left intact, with the nests visibly pulled apart. Based on this evidence, is it likely the depredation is a result of avian or mammalian predators, as opposed to insects (ants) or reptiles (snakes). Potential nest predators include greater roadrunner (Geococcyx californianus), California scrub-jay (Aphelocoma californica), common raven (Corvus corax), and coyote (Canis latrans), which were all observed onsite during the 2017 surveys. Pair B made two successful nesting attempts over the course of the surveys in 2017. Successful nesting was confirmed with the pair tending fledglings nearby, and with dispersing juveniles detected later in the survey visits. Why did one pair succeed (twice) while the other failed (four times!)? Could it be due to the location of the territories? Did one territory have a higher number of predators? Was the health of the food source in the territory poor? Or could it have been due to the maturity of the pair? (younger breeding adults tend to have a lower success rate than more mature adults). As the 2017 survey season wrapped up, we held out hope that one more year of letting the restored site flourish would result in more pairs and more success.
The 2018 survey yielded different results, but not the kind the biologists wanted to see. Pair A was not detected onsite, or within any of the immediately adjacent habitat within audible range of the site during the 2018 surveys. Although it is unclear why Pair A had vanished, it is possible that this pair dispersed to other areas since they were observed to make four unsuccessful nesting attempts on-site last year. Only one pair of gnatcatchers was detected during the 2018 surveys, and this pair’s territory matches the territory of Pair B from the previous year, so they are most likely the same pair.
Pair B made three confirmed unsuccessful nesting attempts over the course of the surveys in 2018, with an unconfirmed, possible fourth attempt. The pair was first observed nest-building in a California sagebrush (Artemisia californica) during the first survey visit, and this nest was likely depredated based on evidence observed during a following survey visit to the territory approximately two weeks later (nest disheveled, pair not tending any nestlings). The pair was then observed to be affiliated with a specific area on the site over the course of three survey visits to the territory, and this may represent a possible unsuccessful nesting attempt. However, no clear evidence of nesting was observed during this period. A new nest in a white sage (Salvia apiana) was observed during the next survey round in a relatively new area for the pair, with the adults exchanging incubation duties. This nest was confirmed to be unsuccessful when the adults were observed tending a brown-headed cowbird (Molothrus ater) fledgling two weeks later. The last confirmed nesting attempt was discovered later that month during the final official survey visit at the end of June, when the pair was discovered incubating a nest in a black sage (Salvia mellifera) high on the slope and in an area very close to 2017 nest locations. This nest was deemed unsuccessful based on evidence observed during a final, passive survey visit two weeks later, which included observing the male and female in highly disjunct areas onsite, and neither individual was observed tending juveniles. It is possible that this pair could have made additional nesting attempts after the final survey visit.
So, how is it that a gorgeously restored mitigation site, with acres upon acres of practically pristine coastal sage scrub protected from human disturbance, supports hardly any successful breeding pairs of gnatcatchers? While we can’t know for sure based on the data we’ve collected so far, we do know a few things that could have potentially contributed to the lower than expected numbers of gnatcatcher pairs for 2018. The San Diego region received 40% of normal rainfall during this past wet season (NWS 2018), and this is likely a contributing factor to the relatively poor breeding season this species had in 2018. Also, we know that brown-headed cowbirds are negatively affecting the resident gnatcatcher pair onsite, and the presence of these nest parasites may drive out prospective breeding pairs from the area. Whatever the case may be, our coastal California gnatcatcher (canary) is telling us that even though the coastal sage scrub (coal mine) may look healthy, we need to dig deeper to find more underlying causes for the breeding failures of this indicator species and implement management strategies to improve the condition of the ecosystem and subsequent success of one of San Diego’s most beloved threatened songbirds.
Photo Credit: By Brennan Mulrooney