Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/41

 It is difficult to do this until we know in the first place what guides the response into certain definite channels. There may even be other factors which might contribute towards the result we see. A suggestion was put forward in the life of the Blackcap that the temporary separation of the sexes, which occurs in the case of many migrants, might lead to an intensification of the response when the crucial moment did at length arrive; and moreover, that it was almost possible to trace a distinct gradation in the visible manifestations, commencing with the species that pair for life and culminating with those which are necessarily separated at the commencement of the period of sexual activity. A considerable body of observations taken from the lives of many species would be required to show whether this suggestion has sufficient foundation of fact. Our comparisons, it may be remembered, showed that the reactions may be alike, or unlike, or differ only in degree. No one could tell whether the bird he were observing was a Grasshopper Warbler or a Savi's Warbler, if he were to judge by attitude alone at a time of sexual excitement; no one on the other hand could mistake a Willow Warbler for a Chiff-chaff under similar circumstances, though he might possibly confuse a Willow Warbler with a Wood Warbler; and so we might draw up a list of closely related forms showing similar irregularities of behaviour. Of the remainder of our more common Warblers, the Blackcap can be distinguished from the Garden Warbler, the Whitethroat from the Lesser Whitethroat, and the Reed Warbler from the Marsh Warbler. The last two species are well worthy of attention, so remarkably alike are they in appearance, and yet so far apart in their power of song and in emotional manifestation. Well may we be excused some feeling of disappointment with our feeble efforts at interpretation. That the difference in the attitudes they assume is one of degree only can be seen by comparing the corresponding plates in the lives of the two species, wherein the wings are represented as partially and fully expanded.