Breeding Home Range in the Nighthawk and Other Birds: Its Evolutionary and Ecological Significance

Nighthawks, Chordeiles minor, lay their eggs on flat surfaces–in cities on flat roofs–roost in trees, and feed on flying insects which in turn presumably feed on vegetation. Thirteen neighboring nighthawk breeding home ranges in the center of Detroit, Michigan included on the average 10.4 ha total area, 97 small trees (including large shrubs), 70 large trees, and 38 flat roofs, representing 2.25 ha. More or less centrally in each home range and preferentially over flat roofs, whether being used for nesting or not, resident males performed diving and booming displays. High average and minimum distances between the centers of activity of the residents, together with observations of conflict between neighbors indicated that the home ranges were mostly defended and, since including feeding and breeding activities, constituted type A territories. Home range size which varied more than threefold, showed no significant correlation with the density of any environmental feature, including an index of photosynthesis, except for a negative correlation with number of flat roofs per ha. Six of the 13 home ranges bounded a 42 ha area unoccupied by nighthawks, which was, compared with the home ranges, deficient in trees and especially in flat roofs. These six home ranges tended to be larger than the remaining seven, though not to differ from them in density of trees and roofs, and in neither group was there correlation between size and density of these features. Thus, variation in home range size seems best explained by birds settling thickly where flat roofs are numerous, and through mutual aggressiveness having small home ranges, by birds not settling at all where flat roofs are scarce, and by home ranges next to such unoccupied areas expanding into them and/or at the expense of neighbors who must defend their entire boundaries. Although nighthawks do not always defend combined nesting and feeding areas, the apparently primary importance of aggressive interaction in determining home range size, in the present study, suggests that in nighthawks, as well as in other species having more strictly type A territories, aggressiveness at high population levels may reduce the proportion of individuals breeding and population natality rate below what resources would allow. A variety of evidence is reviewed on this point including the interspecific variation of home range size in relation to body mass which suggests that bird home ranges generally contain more food resources than required by the occupants.