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New Photo Guide Estimates Osprey Chick Age From Plumage

Thirty-day-old osprey nestling showing developing feathers in its nest
Research · Reproduction & development

A photographic guide published on 15 April 2026 divides osprey (Pandion haliaetus) chick development into 11 plumage stages that can be used to estimate age from nest photographs. The method was developed from chicks of known age photographed in Massachusetts, USA between 2019 and 2022.

The reference material covered chicks from the day of hatching to 47 days old. Researchers used changes in down, contour feathers and flight feathers to define the 11 stages, then made 548 nestling classifications from photographs without consulting the birds’ known ages. A statistical model linked each visually assigned stage to the ages at which that appearance was most likely.

Inexperienced observers matched the authors in 79% of assessments

Five observers who had not developed the guide each classified a random set of 100 photographs. They chose the same stage as the authors in 79% of assessments. In the remaining 21%, they selected an adjacent stage; none of the reported disagreements skipped farther through the developmental sequence.

An associated open library contains 320 photographs of known-age chicks, allowing users to compare several examples across development. The tool is intended for interpreting nest-monitoring photographs when the actual hatch date was not observed directly.

Sources and image credit

  1. Depot, K. M., Poole, A. F., & Purtell, G. (2026). A photographic guide to plumage-based estimation of nestling age in Osprey. Journal of Raptor Research, 60(2).
  2. Depot, K. M., Poole, A. F., & Purtell, G. (2026). Shiny app for interactive photo library of known-age Osprey nestlings (Massachusetts, USA, 2019–2022) [Data set]. KNB.

Image: Thirty-day-old osprey nestling at Allens Pond, Massachusetts, 16 July 2021. Study photo: Depot, Poole & Purtell (2026), KNB, CC BY 4.0. Cropped. · CC BY 4.0 · Source

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