Saturday, June 30, 2012

drought and bird populations, distribution, density

Ph. by Patti McNeal; Wikim. Commons
There has been some discussion recently (on the Wisconsin Birding listserv and the WI Birding Facebook page) regarding the high numbers of Dickcissels found in WI this summer – possibly reflecting drought conditions on the Great Plains. Bird distribution and density are complex phenomena, and there may be several inter-related and interlocking factors driving changes. But there is no doubt that drought affects nesting patterns of birds from multiple avian taxa groups. Here are just a few “hints” at more details:

“Nesting of Vesper Sparrows, Horned Larks and Western Meadowlarks ended abruptly in mid—June 1988 during a period of extremely hot weather. In 1987 and 1989, nesting continued into July. Despite substantial reductions in bird density and productivity during the drought, many species recovered to predrought levels 1 yr following the drought. This suggests that year-to-year fluctuations in densities of some of these species may not be tightly linked to short-term changes in local productivity. However, sequential years of low productivity may have more substantial effects on these short-lived species. Thus, if drought conditions in North American grasslands become more frequent, as some climate models predict, there could be related changes in the avifauna of the region”.

George, T. Luke, Ada C. Fowler, Richard L. Knight, and Lowell C. McEwen. 1992. Impacts of a Severe Drought on Grassland Birds in Western North Dakota. Ecological Applications 2:275–284.

A “study, also published in Global Change Biology, focused on drought, ultimately questioning the wisdom of considering all green space viable bird habitat. The amount of bird refuge is usually measured by the Normalized Difference Vegetation Index (NDVI), which measures the density of green leaves on the ground. However, since green cropland that can’t serve as bird cover during a drought still gets picked up on the index, scientists suspected there could be a better measure of refuge quality. They compared the NDVI data with 15 years of precipitation data from the North American plains. Precipitation proved to be a much more accurate indicator of population health and diversity than the NDVI. But the data also revealed that migratory bird species can experience population losses up to 13 percent during extreme droughts”.

FromHurricanes and Droughts Beat Back Migratory Bird Populations” , by S. Dowdey. Apr 2010

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