This plot shows the number of birds and mammals described per year -- the same as the previous plot, but the data points are removed, and only the smoothing line remains. Again, the Y-axis number in this plot is the natural log (with one (1) added to make the zero years plot) of the number per year. The lines drawn are smooth fits (using an SPlus function "supsmu") of the points.
The plot for mammals initially follows and is slightly below the curve for birds. This seems reasonable, as there are fewer species of mammals than birds.
The curves roughy parallel each other till about 1850, when mammal descriptions level off as the frequency of bird descriptions continues a slow decline. Then a second peak of mammalian description activity is at about 1900-1905. This second peak in mammals is higher than the first. This is an intriguing pattern.
I asked many mammologists if they could explain this second peak; none could. No explanation was found until I sent a copy of the plot to the late Karl Koopman. Karl said the explanation was easy, and in his inimitable style acted as if it should be widely known.
He maintained that snap-traps for collecting small mammals were first used in the United States by workers in the Biological Survey in the late 1800's (1880-1890). An individual from the Biological Survey (whom Karl named, but I now can't remember) went over to England early in the 1900's and took some snap traps with him. The mammologists at the British Museum (most especially the indefatigable Michael Rogers Oldfield Thomas) immediately recoginzed the potential for collecting small mammals.
Collectors from the British Museum and elsewhere subsequently went out all over the world equipped with snap traps with a new and efficient method for collecting small mammals. The second "peak" consists largely of Rodentia, and almost entirely of small mammals -- the increase resulting from a sudden and dramatic improvment in collecting techniques. It can be considered the "snap-trap" effect.
Note: These mammalian data are current only through 1993, though I expect that the apparent recent "upswing" in forms described may well have continued.