Tuesday, February 23, 2010

Drought and Birds: Avocets and Stilts

The complex of wetlands in the Lahontan Valley of Nevada, designated as a Hemispheric Reserve within the Western Hemisphere Shorebird Reserve Network in 1988, declined in area from 34,800 hectares in 1905 to 6,150 hectares in 1987, a net loss of 82 percent.

Julia A R. Alberico (1993) studied the breeding biology of American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus) in the Lahontan Valley in the drought year of 1991 and reported the results in Western Birds.

Her Summary:
I monitored breeding American Avocets and Black-necked Stilts in the Lahontan Valley, Nevada, during the fifth year of drought. There were few sites suitable for breeding, and at sites where birds did breed, nest depredation was extremely high. In a non-drought year there would be thousands of breeding recurvirostrids in the area I monitored; in 1991 there were fewer than 100. Only six pairs of stilts hatched chicks, and avocets failed entirely.

Nest predation pressure is probably higher in drought years because nests are more accessible to coyotes, and duck eggs and other prey items for ravens are limited. Antipredator behaviors seemed ineffective under such pressure.
In areas where 1,000, and often as many as 4,000, young avocets and equal numbers of stilts were routinely produced in each of the 18 years monitored between 1949 and 1975, Alberico documented no avocet chicks and just 21 stilt chicks.

Alberico cites others to the effect that recurvirostrid populations declined in the Lahontin Valley during the dry years of 1976 and 1977, while simultaneously increasing at Great Basin wetlands in Oregon and Utah, "suggesting they had moved from drought-stricken areas such as the Lahontan Valley."

Of 59 avocet and 10 stilt nests at one site (Mahala Slough) in 1991, 13 avocet nests (22 percent) were depredated by coyotes (Canis latrans) and 42 avocet nests (71 percent) and 2 stilt nests (20 percent) were depredated by birds. Common Ravens (Corvus corax) were the primary avian nest predators, although California Gulls (Larus californicus) may also have been involved occasionally.

Alberico comments about the effects of the drought on predation:
Several conditions associated with drought might have increased nest vulnerability and predation rates on recurvirostrid nests in the Lahontan Valley. As Mahala Slough dried up, I observed (from tracks and direct sightings) increasing coyote traffic around nesting areas, coupled with an increase in nest predation by coyotes. As the ponds dried up, nests initiated on hummocks surrounded by water soon became accessible via land or by shallow wading.
More specifically, she found that "Stilt nesting success was related to water depth, as five of six successful nests were surrounded by water deeper than 0.75 m."

Also, "A drought-induced shortage of typical prey items [especially nesting waterfowl] may have prompted ravens to increase their predation on avocet and stilt eggs."

Alberico’s article can be read in full by clicking on the highlighted title below.

Citation:

Alberico, Julie A. R. 1993. Drought and predation cause avocet and stilt breeding failure in Nevada. Western Birds 24: 43-51. [.PDF]

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