Large-Billed Insectivorous Birds: A Precipitous Diversity Gradient

Most hypotheses proposed to account for a latitudinal species-diversity gradient in a particular taxon (e.g., the "time" hypothesis and the "predation" hypothesis, after Pianka 1966) can operate independently of any geographic difference in the abundances of the resources partitioned by the taxon. (For theoretical reviews, see Klopfer 1959; MacArthur 1964, 1969; Whittaker 1965, 1969; Pianka, op. cit.; MacArthur and Wilson 1967; Slobodkin and Sanders 1969; Orians 1969.) An important class of hypotheses, however, postulates that some resource or set of resources, marginal in one area, is increased in abundance in a second area so as to allow more species of a certain kind to evolve and/or persist (MacArthur's category 4, 1969). Implicit in such an hypothesis, of course, is the assumption that at or below some level, productivity limits, or is proportional to, species diversity. To detect such an effect, often a useful procedure is to subdivide the given taxon into groups of species, each largely dependent on a particular kind of resource, and then to look for differences between groups in the steepness of their individual diversity gradients. In this study I have followed such a procedure in an analysis of the insectivorous birds of North and Middle America, a group which shows a marked diversity gradient on a macrogeographic scale. Some of the results have been alluded to previously (Moreau 1966'; Orians 1969), but are here quantitatively documented for the first time. The material below should be considered supplementary to the more extensive data and conclusions of Orians' (1969) study. Excluding families whose members mostly probe or excavate for their prey [Picidae (woodpeckers), Sittidae (nuthatches), Dendrocolaptidae (woodhewers), and Certhidae (creepers)], there are ca. 121 breeding bird species found in the transcontinental belt of North America (42-44' N) whose food is estimated to consist entirely or primarily (> 70 per cent) of arthropods. In contrast, there are ca. 225 such species, somewhat less than twice as many, in the belt 8-10' N traversing Central America, despite the fact that the area of the tropical belt is only about one-seventh that of the tempe ate. The species may be plotted according to the number whose bill lengths fall into e ch of several size categories. When this is done (fig. 1), a striking difference appea s: bill lengths of tropical species are distributed over a much greater range than are those of temperate species (about a 40 per cent extension). Furthermore, the bulk of the tropical species falls over considerably higher values (median, about 16.0 mm) than does hat of the temperate species (median, about 11.8 mm). Indeed, below 14 mm, temperate species are slightly more numerous than tropical ones; it is entirely above this value that the great increase from 42-44' N to 8-10' N in tropical insectivorous bird species diversity takes place. Before speculating on the causes of this pattern, the possibility must be considered that it is merely a consequence of the coarseness of the aereal transects. That is, perhaps if we were to examine areas of several hundred acres, we would find much the same distributions of bills at all latitudes. This would imply that the geographic ranges of large-billed insectivorous birds are relatively smaller than the ranges of small-billed insectivorous birds in the tropics than at higher latitudes. In other words, when moving from one small area to another, faunal turnover for large-billed species would be relatively greater in the tropics, thus transmuting the phenomenon to be explained. One possible way to check for this is to compare geographic ranges. However, because of the possibility of patchiness over the total range, I shall use a different test, that of comparing avifaunas of small areas with regard to bill lengths. Nine tropical and 24 temperate censuses were examined. Of the many possible temperate censuses, I selected those associated with well-known ecological studies (Kendeigh 1946, 1948; Odum 1950; Salt 1957). The tropical censuses include five Panamanian areas from the MacArthur et al.