Visual acuity in an opportunistic raptor, the chimango caracara (Milvago chimango)

• We estimated the visual acuity of the chimango caracara using operant conditioning experiments
• Chimango caracara visual acuity ranges from 15.08 to 39.83 c/deg
• Chimango caracaras have the lowest visual acuity estimated in any raptor to date
• Their relatively lower visual acuity may reflect their foraging on the ground

Raptors are always considered to have an extraordinary resolving power of their eyes (high visual acuity). Nevertheless, raptors differ in their diet and foraging tactics, which could lead to large differences in visual acuity. The visual acuity of an opportunist bird of prey, the Chimango caracara (Mivalgo chimango) was estimated by operant conditioning. Three birds were trained to discriminate two stimuli, a positive grey uniform pattern and a negative grating pattern stimulus. The visual acuity range from 15.08 to 39.83 cycles/degrees. When compared to other birds, they have a higher visual acuity than non-raptorial birds, but they have the lowest visual acuity found in bird of prey so far. We discuss this result in the context of the ecology of the bird, with special focus on it is foraging tactic.