Trophic isolation in birds depends on their varying ability to find their food (prey on the wing, or at rest on a perch, or on the ground). This paper discusses the part played by wing-length and body-weight in this process of isolation. A coefficient relating these two morphological factors, Wing-length/Logarithm of body weight, is suggested. The value of this coefficient varies in relation to the diet and the density of the layers of vegetation in which food is found. It increases as the diet becomes richer in insects and as vegetation in the layer where food is found gets thinner. This coefficient is thus a tentative way of measuring the range of hunting ability typical of each species.