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Habitat Distribution and Territoriality In the Dickcissel and Red-Winged Blackbird

Summary


The dickcissel and red-winged blackbird are two common, territorial bird species occurring throughout the central U.S. Both commonly inhabit fields and roadside edges. The redwing is also very common in marshland.

The dickcissel inhabits oldfield habitats at considerably higher densities than prairie habitats. Relative suitability of these habitats as determined by breeding success and sex ratios is higher in the oldfield than on the prairie. Sex ratios, and hence, suitabilities, appear to rise with increasing male densities, probably reflecting an increase in vegetational volume over time.

Territory size in the dickcissel is inversely correlated with male density. A minimum territory size of about 0.35 acres was reached at a density of about 105 males/100 acres. A minimum territory size imposes a limit on the density and it is presumed that unsettled males are forced into less suitable habitats at high densities. The minimum territory size also implies that some males in the same habitat are forced to include less suitable vegetation in their territories at densities above those at which the minimum territory size is reached.

On the basis of suitabilities, densities, and territory size dynamics, it is concluded that the habitat distribution of the dickcissel approximates the ideal dominance distribution model described by Fretwell and Lucas (1969), and that territorial behavior in the dickcissel functions to limit density in local populations.

The red-winged blackbird occupies marsh habitats at very high densities compared to upland (oldfield and prairie) habitats. These habitats do not appear to differ in suitability for redwings when occupied by breeding populations.

Sex ratios decrease with increasing density (and decreasing male territory size), probably reflecting territorial behavior by females which is believed to limit the number of females settling in any one male's territory. Redwing sex ratios are not believed to be valid indices of habitat suitability, in light of female territorial behavior.

Territory sizes in male redwings are inversely correlated with male density but show no apparent minimum size. Hence, there is no real limit placed on density and unsettled males cannot be said to be forced into less suitable habitats.

Territory size dynamics, comparative densities, and relative suitabilities all support the conclusion that the redwing habitat distribution is ideal free, not ideal dominance. Male territorial behavior thus functions primarily as a density cue for unsettled birds and does not effectively limit population densities.


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