Bird population trends from constant effort mist netting in Oregon Caves National Monument and Preserve

Rockwell, Sarah; Alexander, John; Stephens, Jaime; Frey, Robert

As part of the NPS Klamath Network Inventory and Monitoring program, we monitored bird populations from 2002-2013 in a high elevation riparian habitat surrounded by mature coniferous forest at Oregon Caves National Monument and Preserve (ORCA), using standard mist netting and banding methods. We investigated trends in abundance of adults and young for 10 species of breeding, fall migrant, and year-round resident birds. We then compared population change at ORCA with estimates derived from the Breeding Bird Survey (BBS) Northern Pacific Rainforest region, and examined productivity as a possible cause of population change. We found the strongest evidence for declines in adults and/or young for MacGillivray’s Warbler (Geothlypis tolmei), followed by Western (Pacific-Slope) Flycatcher (Empidonax difficilis). MacGillivray’s Warbler was the only species where declines in both the local breeding population of adults and young were revealed, with corresponding declines in the regional BBS data. We further assessed that species’ regional status by examining patterns at nine additional banding stations, and found a decreasing trend in breeding adults for just one other site, also at high elevation. Fall populations of Wilson’s Warbler (Cardellina pusilla) adults remained stable and abundance of young increased, contrasting with a negative trend found for breeding adults in the BBS data. Adult populations of other fall migrants and all year-round residents studied remained relatively stable during this period, despite significant declines in Golden-crowned Kinglet (Regulus satrapa) in the BBS data. Capture rates of local young Western Flycatcher declined over the study period, while regionally produced Oregon Junco (Junco hyemalis oreganus) young increased. Capture rates of young in one year were not associated with adult abundance in the following year for any species; thus, we found little evidence that local productivity is the driver of population change at ORCA. A greater understanding of the conservation biology of these species is needed, because some may be undergoing declines that we do not have sufficient sample sizes to detect. Further research into the potential climatic and anthropogenic causes of observed population trends and their proximate demographic mechanisms in all phases of the annual cycle are needed to understand and address causes of declines.

Bird population trends from constant effort mist netting in Oregon Caves National Monument and Preserve

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