Northern Prairie Wildlife Research Center
The concept of suitability regarding these two species is also interesting. In the dickcissel, suitability was seen to rise with density and differed between habitats. This was seen to fit the ideal dominance model even though one of the assumptions of the model was invalid. In the redwing, it was not possible to plot suitability vs. density for any one habitat. But since the ideal free model is fit quite nicely by the redwing, it would appear as though suitability decreases with increased density, according to the model. Thus, even though maximum suitabilities of different habitats may vary, suitabilities will be equal when all habitats are occupied by the redwing at different densities.
One wonders why dickcissels and redwings have different territorial systems even though they occupy some of the same habitats. Brown (1964) says that the type of territoriality evolved in a species depends on the types of requisites for which competition exists. The fact that the redwing and dickcissel employ different systems implies that the primary requisites for competitive or aggressive territorial behavior are different in the two species. Just what these requisites may be for each species is an open question. It seems reasonable to assume that competition would exist in both species for such basic things as breeding opportunities, food resources, and just plain living space. It is possible, then, that the dickcissel and redwing have different priorities for these requirements.