Trends in landbird abundance at Channel Islands National Park, 1993-2015

Coonan, Timothy; Dye, Linda

The park’s landbird monitoring program, one of the longest-running landbird population monitoring programs in the U.S., provided trend data for 29 of the park’s 48 known breeding landbird species. Density estimates from distance sampling were constructed for 19 species. Distance estimates were not available for the legacy point count sites on Santa Cruz, but we used point count data to determine trends for 26 species on that island. Similarly, count data were also used to derive trends for 7 species on other islands. The methods were incapable of detecting trends for relatively rare species such as swifts and swallows, owls, and some large raptors such as bald and golden eagles and peregrine falcons. Distance sampling methods allow estimation of density and are robust to observer variability, due to incorporation of covariates in the model. Problems with confidence intervals and annual variability are lessened over time, as the number of detections for each species accrues. Trend analysis revealed various rates of change in landbird populations on the five park islands. While small sample sizes made conclusions about change on Anacapa tenuous, there was less change, overall, on Santa Barbara and San Miguel, the other two smaller islands. Some trends on the latter seemed related to the decline and recovery of the island fox, an effective nest predator. The relatively low amount of change on those islands echoes concerns that recovery of vegetation from past land use practices, such as grazing, has slowed or stalled (Davidson et al. in review). There was much greater change on the two larger islands of Santa Cruz and Santa Rosa, where recent removal of nonnative ungulates has allowed incipient recovery of shrub and woodland communities. The removal of domestic sheep and feral pigs from Santa Cruz Island, in the 1980s and 2000s, respectively, has apparently resulted in the increase of landbird species which prefer shrub and woodland habitat, and a decrease in those that prefer grassland. There was less change on Santa Rosa, where cattle were removed in 1998, elk and most deer by December 2011. Still, increases in some shrub and woodland bird species on that island suggest that Santa Rosa may eventually follow a similar recovery track as Santa Cruz, where almost all landbird species experienced change over time.

Trends in landbird abundance at Channel Islands National Park, 1993-2015

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