Birds that feed-on-the-wing have few isthmo-optic neurons.

The isthmo-optic system is less developed in birds feeding-on-the-wing, than in pecking avians. This was suggested previously. By intraocular horseradish peroxidase applications, we studied the central origin of this retino-petal system in thrush, haw finch, swift and swallow. Our data support the assumption on a correlation between feeding habits and the development of the isthmo-optic nucleus in adult avians as this brainstem region is more highly developed in thrush and finch than in swift and swallow. This is particularly relevant since the latter species is taxonomically related to the two pecking birds whereas it is unrelated to the swift that also feeds-on-the-wing.