Avian Prehatching Behavior: Functional Aspects of the Tucking Pattern

Abstract

The domestic chick's position at the time of pipping (initial cracking of the eggshell) is important to its hatching success. Normally the anterior end of the chick is at the large end of the egg; the legs, feet, and left wing are flexed; the neck is depressed ventrally; the head is turned to the right, its left side resting on the right pectoral muscle, and is tucked under the partially extended right wing (Fig. la). Six malpositions generally reducing hatchability have been recognized by poultry scientists. Malposition VI, head over the right wing (Fig. lb), reduces hatching, but so slightly that Waters (1935), Byerly and Olsen (1937), Munday (1953) and others did not consider this position abnormal. Hamburger and Oppenheim (1967) detailed the specific prehatching pattern of tucking, the placing of the head under the right wing 1-2 days before pipping. This attitude is maintained throughout the hatching stage of cutting the cap from the shell. It is a genetically consistent behavior pattern of the chick and probably of most birds (see Oppenhelm 1972). Thus I was interested to learn that its absence (producing malposition VI) does not seriously hamper pipping or hatching. This could be explained by the fact that only a very slight selective advantage is necessary to fix an adaptation in a species over a large number of generations, but it still remains to elucidate the function(s) of the wing and the actual selective advantage produced by tucking. What is the function of the normal wing-overhead position? Kuo's (1932) supposition that it is important for the wing to protect the face from the air chamber membrane has been shown invalid by Waters (1935) and Hamburger and Oppenheim (1967). Protection of the head or eyes from sharp shell fragments during the head thrusts of climax could be a function, but the eyes generally remain closed until hatching is completed in precocial birds (Hamburger and Oppenheim 1967), longer in altricial birds, and the eyes may need no extra protection. Also, Oppenheim (1972) stated that the wing in altricial birds does not cover much of the relatively large head. Normal pipping convulsions (Brooks and Garrett 1970) might deform the delicate wing bones if the wing were between thorax and beak in malposition VI. Wing protection is not needed in the hatching climax, however: the thorax does not push against the beak, now, because there is sufficient space in which to elevate the head; and the force does not have to be as great to break the eggshell (which is already broken) further. Yet the wing covers the head throughout. Narayanan and Oppenheim (1968) hypothesized that tucking inhibits head movements. In their experiments involving extirpation of the right wing bud, a significant increase in frequency of head movements was demonstrated, but the cause of the increase was obscured by possible neural modifications due to limb extirpation. Later Oppenheim (1970) re-tested this by manually un-tucking chicks and ducklings, but no significant increase in frequency of head movements was shown this time. In any case, it seems doubtful that a decrease in frequency (rather than amplitude) of head movements would produce the selective pressure necessary to evolve the tucking pattern. In fact, the opposite might be more beneficial.