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 intraocula r horseradish peroxidase applications, we studied the central origin o f this retino-petal system in thrush, haw finch, swift and swallow. Ou r data support the assumption on a correlation between feeding habits and the development of the isthmo-optic nucleus in adult avians as thi s brainstem region is more highly developed in thrush and finch than i n swift and swallow. This is particularly relevant since the latter sp ecies is taxonomically related to the two pecking birds whereas it is unrelated to the swift that also feeds-on-the-wing.