Degree Type


Date of Award


Degree Name

Master of Science


Animal Ecology


Animal Ecology

First Advisor

Louis B. Best


The Dickcissel (Spiza americana) is well known for its erratic, annual shifts in distribution and abundance and irruptive behavior on the breeding grounds. Annual fluctuations in the species' breeding range have been purported to be related to fluctuations in precipitation and its influence on annual vegetation growth (e.g., Roth 1979). A recent hypothesis suggests that mowing of hay fields in the center of the species' range also may contribute to opportunistic, drought-related movements to extralimital portions of the species' range (Mulvihi111989). Mowing seems to loosen or disrupt the between-year site faithfulness that characterizes relatively stable habitats. Low reproductive success and low annual return rates in alfalfa suggest that Dickcissel populations in regularly-mowed alfalfa fields can be maintained only by continued annual immigration from source habitats. Recolonization of alfalfa fields after mowing by Dickcissels in the drought year 1988 was poor. A positive relationship between Dickcissel densities and April soil moisture in our alfalfa fields and a negative relationship at the edge of the species' range suggest that mechanisms at the regional level may be an extension of interactions taking place at the local level. Earlier mowing may contribute to Dickcissel movements to the edge of the species' range.



Iowa State University Digital Repository

Copyright Owner

Lawrence D. Igl



File Format


File Size

57 pages