Wednesday, August 8, 2007

PS 55-205: Avian insectivores respond to 17-year Brood X periodical cicada emergences

V. Beth Kuser Olsen, University of Maryland and Cecil Community College

Every 17 years, Brood X of the North American periodical cicada, Magicicada species, emerges from its underground life as larvae, temporarily offering an unlimited food resource for insectivores. Using the USGS Patuxent Wildlife Research Center’s Breeding Bird Survey (BBS) 40-year database, I measured the effects of this 17-year cycle on the abundance of 14 selected avian insectivores.  I predicted these breeding bird populations would respond to Brood X with an increase in abundance the year following cicada emergences.  I combined BBS bird counts for three Brood X emergences, 1970, 1987, and 2004.  Using one-tailed Wilcoxon signed ranking non-parametric tests on log-transformed, de-trended data, I compared avian abundance the year prior to an emergence to each of the four years following an emergence.  Of the 14 species, 5 showed significant increases in abundance in the year following an emergence: Yellow-shafted Flicker, Colaptes auratus auratus, Great Crested Flycatcher, Myiarchus crinitus, Brown Thrasher, Toxostoma rufum, Eastern Towhee, Pipilo erythrophthalmus, and Baltimore Oriole, Icterus galbula.  None sustained this significant increase beyond the year following the Brood X emergence.  This correlation suggests these 5 species make use of the cicadas as a major food source during emergence years, temporarily boosting population size the following year.  The other 9 species predicted to use the cicadas as a main food source did not show a significant increase in population size correlated with cicada emergences.  This suggests either these species do not make use of cicadas as a resource or the methods of analysis did not detect the response.   

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