Field Experiments On the Adaptive Significance of Avian Eggshell Pigmentation

Abstract

A series of four field experiments which tested the camouflage properties of different eggshell pigmentation and marking patterns was conducted. Neither natural (Laughing Gull, Larus atricilla, eggs) nor artificial (khaki, khaki-randomly spotted, and black painted eggs) camouflage patterns conferred any selective advantage, when crows (Corvus brachyrhynchos and C. ossifragus) were allowed extended periods (over 2 hours) to hunt for the eggs. However, a natural eggshell pattern (Japanese Quail, Coturnix c. japonica, eggs) conferred a very strong survival advantage during field tests which were terminated either after half the eggs were discovered or after I hour, whichever came first. The selection index (SI) obtained for the quail eggs was higher than those yielded from other experimental investigations of the survival value of camouflage. The results of these experiments were discussed in view of the different test procedures employed, and the extent to which such procedures are reflective of naturally occuring patterns of crow predation, such as in a gull or a tern colony. A fifth experiment which tested the capacity of pigmented and unpigmented eggshells to shield egg contents from solar radiation was also conducted. Pigmented eggshells provided far less effective solar shielding than did unpigmented ones, and this effect appears to hold over a wide range of eggshell thicknesses. In view of these findings, the dark eggshell pigmentation patterns of open ground nesting birds are seen as an adaptive compromise. That is, under different circumstances dark eggshell pigmentation may be either advantageous or disadvantageous to species and individuals which possess them. Based upon functional considerations an hypothesis was generated which predicts that natural selection should favor darker eggshell pigmentation patterns among certain avian species with particular types of reproductive strategies nesting in particular types of breeding habitat. Some general evidence in support of this hypothesis was offered, and some suggestions for further comparative tests of this hypothesis are suggested.