352 TROPICAL WILD LIFE IN BRITISH GUIANA 



different portions of the wing. Among the birds that are 

 fairly strong fliers, when one segment increases proportion- 

 ally in length, another decreases in the same proportion, or 

 both of the others diminish to such an extent that the sum of 

 their total loss equals the gain of the first. Furthermore, 

 when there is a local change in one segment at a single period 

 of growth, it is usually balanced by an opposite change in 

 one of the other segments, or by the sum of the changes in 

 both of the others. Thus in the hoatzin, the toucan and the 

 cacique three widely separated birds we find that the 

 balance is kept in all stages, though the proportionate lengths 

 may greatly differ. 



The blue-winged parrakeet is an exception to the gen- 

 eral rule. In this bird both the arm and the hand increase 

 in length as the fledgling grows older, while the forearm 

 remains approximately the same. On the seventh day, how- 

 ever, the forearm shows a considerable increase over that of 

 the adult and the hand a much greater decrease. While this 

 does not bear out the second rule, it at least shows that there 

 is a tendency toward it. 



In some birds that use their feet more than their wings, 

 the proportional growth of one segment remains about the 

 same, while the others grow in opposite directions. In the 

 heron it is the pinion; in the trumpeter, the arm; in the ja- 

 cana, the forearm. The growth of the other two segments 

 is opposite in each, the greatest variation taking place in the 

 hand and the arm of the jacana. 



Or more concretely : 



A. Throughout the development of the wing of the 

 hoatzin, Guiana green heron, trumpeter and jacana birds 

 that use their feet more than their wings the proportional 

 length of one segment remains constant, while the other two 

 vary in opposite directions. 



B. Throughout the development of the wing in at least 

 one toucan and in several Passeres, a balance is kept between 

 the different segments so that when one portion changes, 



