6o THE SENSES OF ANIMALS 



of what appears to be simultaneous spotting, but which is 

 really an instinctive reaction stimulated by the behaviour of 

 one vulture, or possibly more than one vulture. The fact that 

 this is a kind of follow-my-leader process does not in any way 

 detract from the visual acuity of vultures — quite the reverse. 

 Any one of the flock may have been the first spotter, and to do 

 this from hundreds of feet up is sharpness of sight with a 

 vengeance. 



Perhaps the most interesting of birds to study from the 

 point of view of their eyesight are the owls. Our own species 

 are not all completely nocturnal, but all can hunt in light so 

 dim that one can scarcely credit that their eyes could per- 

 ceive anything. We have five species of owls in Britain : the 

 Tawny or Brown Owl, the Barn Owl, the Short-eared Owl, 

 the Long-eared Owl and the introduced Little Owl. Of these 

 the Long-eared Owl is the most nocturnal, followed closely 

 by the Tawny Owl. The Barn Owl is more nocturnal than 

 not, but it is often seen hunting in daylight — particularly in 

 winter months. The Short-eared Owl is mainly diurnal, but 

 can also hunt in dim light; while the Little Owl seems in- 

 different to the time of day, hunting at all hours depending 

 much upon the season of the year and the prey most easily 

 available at the time. (See Plate 3.) 



These variations in hours of hunting naturally entail dif- 

 ferences in the internal structures of the eyes. Some references 

 to the size and nerve-cells of the eyes of owls have already 

 been made earlier on, and it is only necessary to remind the 

 student that whereas no owl can see in utter darkness, all of 

 them can see well enough in daylight to enable them to fly 

 to a tree and land on it safely. The Long-eared and Tawny 

 Owl will seem a little unsteady in flight if the day is very 

 bright ; and if closely observed, it will be noted that they do 

 not land with the precision which marks their activity at 

 night. No such uncertainty is seen when watching the Little 

 Owl or the Short-eared Owl — nor even the Barn Owl — when 

 these fly in daylight. In fact, it can be stated that the first 

 two of these three species are indifferent to the degree of 

 brightness prevailing. I have seen Short-eared Owls hunting 

 at midday in the height of summer. What must be remem 

 bered is that the relative number of rods and cones in th 



