Birds migrate great distances each year and the primary cue for migration are changes in day length. these changes in day length in turn trigger changes in the birds. behavior and body changes follow as the birds prepare for their seasonal migrations.

answer: 20 percent

the fish culture has become a developing commercial practice in developing countries which is already prominent in developed countries of the world. the major fish protein demand is being supplemented in developing countries is   fishing to meet the needs of the growing population. therefore, in some developing nations, the percentage of animal protein from fish in the diet can be as high as 20 percent.