[Note: this spiel was written quite some time back, but I thought you might find it interesting all the same! -- Jen]
In 2009 through 2011, Great Basin Bird Observatory teamed with Nevada Department of Wildlife (NDOW) to survey colonial waterbirds throughout the state, including White-faced Ibis, as a part of a regional evaluation led by the U.S. Fish and Wildlife Service and American Bird Conservancy. White-faced Ibis, dark-colored wading birds, have a long downwardly-curved bill with which they probe through the mud and debris of wetlands and agricultural fields in search of their prey – in the Great Basin, they feed mostly on earthworms and insect larvae.
In the 1960s and 1970s, pesticides and habitat loss caused the ibis population to crash, across their range[i]. However, in the 1980s and 1990s, they began increasing again; now, the Intermountain West population is considered to be of only moderate conservation priority[ii]. While the regional trend is stable or increasing, there is concern that Nevada’s population may again be declining[iii]. Evaluating White-faced Ibis populations on a state-wide level can be difficult, however, since they are nomadic, with colonies moving as water levels fluctuate.
The Lahontan Valley, particularly Carson Lake, has traditionally been one of the most important White-faced Ibis breeding areas within the Great Basin, and in high water years supports the bulk of Nevada’s breeding population. Breeding numbers vary dramatically here, from 0 (e.g., 1974) to 9000 pairs (e.g., 1997)[iv]. Our results supported this pattern: in 2009, a low water year, NDOW recorded 100 breeding ibis; with the higher waters seen in 2011, 6940 breeding ibis were recorded. The Humboldt River, Ruby Valley, and Quinn River also supported large numbers of breeding ibis, and formed the bulk of Nevada’s population during low water years in the Lahontan Valley.
While pesticide impacts have lessened in past years for the majority of the Intermountain West’s population, eggshell thinning as a result of DDT (and DDE, its metabolic byproduct) remains an important problem for Carson Lake’s ibis, particularly for those breeding early in the season. While DDT was banned from use within the United States in 1972, DDE levels in Carson Lake ibis eggs have remained high over time[v]. Michael Yates and others[vi] reported that many of Carson Lake’s ibis winter in the Mexicali and Imperial Valleys of Mexico and California, and that these birds had high levels of DDE. Unfortunately, White-faced Ibis appear to be physiologically susceptible to DDE/DDT, and their life history increases their chance of exposure: they tend to feed in agricultural fields that may have been subjected to past pesticide applications, and their favored prey are earthworms, which tend to bioaccumulate DDE from soils. In addition, they tend to breed shortly after arriving on their breeding grounds, while their systems still contain DDE, which may explain why earlier nesting individuals tend to have greater eggshell thinning – individuals nesting later have been able to feed on uncontaminated prey, and their DDE levels and eggshell thinning are consequently reduced.
While our joint colonial waterbird project is over, NDOW continues to regularly survey the major White-faced Ibis colonies, including those in the Lahontan Valley, and we continue to maintain our statewide colonial waterbird database. If you have information on breeding ibis colonies (or other colonial waterbirds), locations and/or numbers, we’d love to hear from you! Many thanks to our partners at Nevada Department of Wildlife for sharing their survey results for this project, especially Brad Bauman, Pete Bradley, Jenni Jeffers, and Larry Neel.
[i] Ryder, Ronald A. and David E. Manry. 1994. White-faced Ibis (Plegadis chihi), No. 130. The Birds of North America (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology.
[ii] Ivey, Gary, and Caroline P. Herziger. 2006. Intermountain West Waterbird Conservation Plan. Version 1.2. February 2006. A plan associated with the Waterbird Conservation for the Americas Initiative. Published by U.S. Fish and Wildlife Service Pacific Region, Portland, Oregon.
[iii] Great Basin Bird Observatory. 2010. Nevada Comprehensive Bird Conservation Plan, Version 1.0. Great Basin Bird Observatory, Reno, NV. Available online at www.gbbo.org/bird_conservation_plan.html
[iv] Ivey and Herziger 2006; Oring, Lewis W, Larry Neel, and Kay E Oring. 2001. Intermountain West Shorebird Plan. Version 1.0.
[v] Henny, Charles J. 1997. DDE still high in White-faced Ibis eggs from Carson Lake, Nevada. Colonial Waterbirds 20(3): 478-484.
[vi] Michael A. Yates, Mark R. Fuller, Charles J. Henny, William S. Seegar, Jaqueline Garcia. 2010. Wintering area DDE source to migratory white-faced ibis revealed by satellite telemetry and prey sampling. Ecotoxicology 19:1, 153-162.