Certain rapacious birds who are awkward in hunting, especially Kites, make up for their lack of skill by audacious impudence. Constantly on the watch for better hunters like the Falcon, they throw themselves on him as soon as he has seized his prey. The proud bird, though much more courageous, stronger, and more skilful than these thieves, usually abandons the prey either because the burden embarrasses him in the struggle, or else because he knows that he can easily find another. These highway robbers of the air often unite to gain possession of a prey already taken and killed, and ready to be eaten. A handsome Falcon of the Southern States of North America, the Caracara Eagle (_Polyborus cheriway_), frequently steals fish from the Brown Pelicans on the coast of Texas. When the Pelicans are returning from their expeditions with pouches filled with fish, the Caracaras attack them until they disgorge, and then alight to devour the stolen prey. They do not attack the outgoing birds, but only the incoming ones, and they wait until they reach the land (so that the contents of the pouches may not fall into the water) before pouncing on them.[25] [25] Bendire, _Life Histories of North American Birds_, p. 315. Among other animals a habit has been formed from some special circumstance. As an extreme case in this group we meet with parasites of whom some cannot live outside a particular nest, and are even absolutely transformed by this kind of life. But between these and independent hunters there are an extreme number of intermediate stages, of which it is sufficient to mention a few.[26] [26] For a discussion of this subject, see P. van Beneden, _Commensaux et Parasites_, Paris, 1875. The _Fierasfer_, a little fish of the Mediterranean, installs himself in the respiratory cavity of a Holothurian; he does not live at the expense of his host's flesh, but contents himself with levying a tax on the foods which enter the cavity. It is a case of commensalism of which there are very numerous examples. Other cases may be mentioned which are still further removed from parasitism. Among these may be mentioned the birds who relieve large mammals of their vermin.