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Wilson Bull., 109(2), 1997, pp. 339-343
Response of six species of Hawaiian forest birds to a 1991-1992 El Nino drought.El Nino-Southern Oscillation events (ENSO) are known to effect reproduction and survival of various bird species and other animals (Barber and Chavez 1983, Grant and Grant 1987, Hall et al. 1988, Massey et al. 1992). Although severe storms resulting from ENSO can directly cause bird mortality, primary effects appear to be influences of weather on food resources. During ENSO, precipitation is enhanced in some regions and diminished in others (Philander 1992). Enhanced rainfall may cause abundant seed crops and high arthropod populations, resulting in prolonged breeding seasons for some bird species (Gibbs and Grant 1987) or, along with reduced temperatures, decreases invertebrate populations resulting in breeding failure and reduced adult survival in other bird species (Miskelly 1990).
In Hawaii, ENSO generally causes rainfall to diminish (Philander 1992). Between 1941 and 1983, Hawaii has experienced six major ENSO droughts (Chu 1989). However, despite the periodic occurrence of ENSO droughts, few data are available about the effects on Hawaiian bird species. Here, we report a short-term effect of the 1991-1992 ENSO drought on capture rates, fat scores, and active nests of six resident Hawaiian forest bird species.
Study area and methods.-We conducted this study on the Island of Hawaii in the dryland mamane (Sophora chrysophylla)-naio (Myoporum sandwicense) forest near Puu Laau (19(deg)50'N, 155(deg)35'W) on the southwestern slope of Mauna Kea. Vegetation in the study area was described by van Riper (1980a) and Scowcroft and Giffin (1983). In 1991-1992, an ENSO throughout the Hawaiian Islands chain resulted in a nine-month drought from October through June. Total rainfall (<67 mm) in our study area was approximately 17% of average (Juvik et al. 1993; NBS, unpubl. data). The result of this drought was a general failure of flower and mamane seed set in our study area during 1992 (NBS, unpubl. data). Green seeds from mamane pods are a major food source for the endangered Palila (Loxioides bailleui); birds also use the nectar and flowers of mamane for food.
From April through September 1991-1993, we operated 10 mist nets at each of four fixed banding stations to capture birds within the study area. Nets were opened 1-4 days monthly at each banding...