Vision and touch in relation to foraging and predator detection: insightful contrasts between a plover and a sandpiper.

Visual fields were determined in two species of shorebirds (Charadriiformes) whose foraging is guided primarily by different sources of information: red knots (Calidris canutus, tactile foragers) and European golden plovers (Pluvialis apricaria, visual foragers). The visual fields of both species showed features that are found in a wide range of birds whose foraging involves precision pecking or lunging at food items. Surprisingly, red knots did not show comprehensive panoramic vision as found in some other tactile feeders; they have a binocular field surrounding the bill and a substantial blind area behind the head. We argue that this is because knots switch to more visually guided foraging on their breeding grounds. However, this visual field topography leaves them vulnerable to predation, especially when using tactile foraging in non-breeding locations where predation by falcons is an important selection factor. Golden plovers use visually guided foraging throughout the year, and so it is not surprising that they have precision-pecking frontal visual fields. However, they often feed at night and this is associated with relatively large eyes. These are anchored in the skull by a wing of bone extending from the dorsal perimeter of each orbit; a skeletal structure previously unreported in birds and which we have named 'supraorbital aliform bone', Os supraorbitale aliforme. The larger eyes and their associated supraorbital wings result in a wide blind area above the head, which may leave these plovers particularly vulnerable to predation. Thus, in these two shorebirds, we see clear examples of the trade-off between the two key functions of visual fields: (i) the detection of predators remote from the animal and (ii) the control of accurate behaviours, such as the procurement of food items, at close quarters.