The Visual Acuity of Birds

eagle-6

It turns out that the “little owl” has a visual acuity of about 20/100 whereas the “wedge-tailed eagle” has a visual acuity of about 20/4.  Thus, the wedge-tailed eagle can see at 20 feet what a human with normal vision can see at 4 feet.

To convert cycles / degree (c/deg) to its Snellen equivalent, divide 600 by the c/deg value to obtain the denominator of the Snellen equivalent, the numerator being 20.

From http://www.sciencedirect.com/science/article/pii/S0042698903003043:

Species Acuity (c/deg) Reference
Little owl (Athene noctua) 6 Porciatti, Fontanesi, and Bagnoli (1989) and Porciatti et al. (1990)
Quail (Coturnix coturnix japonica) 6.8 Hodos, Miller, and Fite (1991) and Hodos et al. (1991)
Great Horned owl (Bubo virginianus) 6–7.5 Fite (1973)
Domestic chick (Gallus domesticus) 8 days 7.7–8.6 Schmid and Wildsoet (1998)
Tawny owl (Strix aluco) 8 Martin and Gordon (1974)
Barn owl (Tyto alba) 8a Wathey and Pettigrew (1989)
Blue Jay (Cyanocitta cristata) 15–19 Fite and Rosenfield-Wessels (1975)
Pigeon (Columba livia) 18 Porciatti et al. (1991)
Sacred kingfisher (Halcyon sancta) 26a Moroney and Pettigrew (1987)
Rook (Corvus frugilegus) 30 Dąbrowska (1975)
Jay (Garrulus glandarius) (n=1) 30 Dąbrowska (1975)
Jackdaw (Coloeus monedula) 30–33 Dąbrowska (1975)
Magpie (Pica pica) 30–33 Dąbrowska (1975)
Laughing kookaburra (Dalceo gigas) 41a Moroney and Pettigrew (1987)
American kestrel (Falco sparverius) (n=1) 40 Hirsch (1982)
American kestrel (Falco sparverius) (n=1) 160 Fox et al. (1976)
American kestrel (Falco sparverius) (n=9) 39.7–71.4 Gaffney and Hodos, present report
Brown falcon (Falco berigora) (n=1) 73 Reymond (1987)
African serpent eagle (Dryotriorchus spectabilis) (n=1) 120b Schlaer (1972)
Wedge-tailed eagle (Aquila audax) (n=1) 132–143 Reymond (1985)