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Common Kestrel Retina Had Two Deep, Cone-Dominated Foveal Pits

Full head of a common kestrel with the eye in clear focus
Research · Anatomy & morphology

A detailed anatomical study of the common kestrel (Falco tinnunculus) found two distinct deep foveae in the retina and marked structural differences from the centre to the periphery. A fovea is a specialised retinal depression whose structure is typically associated with very high spatial resolution. The work was published on 17 June 2026 in Biology.

The researchers examined the eyes of two adult male kestrels that became available through the Santa Faz Wildlife Recovery Centre after severe injuries unrelated to the eyes made rehabilitation and release impossible. The birds were euthanised on veterinary grounds rather than for the study, and no gross ocular abnormalities were found. The team measured the eyes, prepared histological sections of different retinal regions and used immunolabelling to examine photoreceptor opsins in the foveae.

The second fovea was also unusually deep

The central retina was the thickest region measured, at about 254 µm compared with about 109 µm in the peripheral retina. Two specialised foveae were identified. The central fovea lay within the area centralis and was about 218 µm deep, while the temporal fovea was about 106 µm deep and also had steep walls. In many bifoveate raptors the second, temporal fovea is shallower, so the pronounced temporal pit is a notable feature of the kestrel retina. Both foveae showed centrifugal displacement of the inner retinal layers around the pit, which reduces tissue in the path of incoming light and is characteristic of retinal regions specialised for high spatial resolution.

Histological sections of the central and temporal foveae in the common kestrel retina
Central and temporal foveae of the common kestrel retina. Figure 3 from Cobo et al. (2026), CC BY 4.0.

Both foveal pits were cone-dominated

Both foveae showed immunoreactivity for S- and L/M-opsins, visual pigments associated with cone photoreceptors that support colour vision and bright-light vision. Rhodopsin, the visual pigment characteristic of rods used especially in dim light, was not detected within the pit of either fovea, although rhodopsin-positive cells reappeared outside the pits. The authors therefore interpret both foveal pits as predominantly cone-dominated regions consistent with high-acuity daylight vision.

The study did not measure visual acuity behaviourally, nor did it directly count cone or ganglion-cell density. The functional interpretation rests on the anatomy of the deep foveae, displacement of inner retinal layers and the photoreceptor markers detected there.

The study provides a detailed morphological and histological baseline for the kestrel visual system. The authors note that only two specimens were available, but the results add species-specific information on how the bifoveate retina of a diurnal falcon is organised.

Sources and image credit

  1. Cobo, R., Jiménez-Díaz, D., Navarro-Sempere, A., García, M., & Segovia, Y. (2026). Eye morphology, foveal structure and photoreceptor composition in both foveae of Common Kestrel (Falco tinnunculus Linnaeus, 1758). Biology, 15(12), 949. https://doi.org/10.3390/biology15120949Primary open-access research article, first published 17 June 2026 under CC BY 4.0. Two adult male common kestrels were examined morphologically and histologically; the study documented two foveae, retinal thickness gradients and photoreceptor opsin distribution.
  2. PubMed record — Cobo et al. (2026)Independent bibliographic record confirming the 17 June 2026 electronic publication date and article identifiers.

Image: P. María-Mojica / Santa Faz Wildlife Recovery Centre, reproduced in Cobo et al. (2026), Figure 1A — common kestrel head and eye from one of the two study birds; cropped to retain the full head while keeping the eye prominent. · CC BY 4.0 · Source

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