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Ospreys Produced Far Fewer Young in Saltier Parts of Chesapeake Bay

Adult osprey and young on a nest at Solomons Island, Maryland
Research · Reproduction & development

Ospreys (Pandion haliaetus) breeding in saltier parts of Chesapeake Bay produced far fewer young than pairs at two low-salinity reference areas in 2024. The study, published 8 June 2026, monitored 571 occupied territories: 487 pairs across ten areas averaging more than 10 parts per thousand salinity and 84 pairs at two areas below 5 parts per thousand. Each focal nest was checked at least four times through the breeding season, with adults, eggs and young recorded to follow breeding attempts towards fledging. Reproductive rate — the number of young fledged per occupied pair — averaged 0.51 in the high-salinity areas and 1.36 at the low-salinity sites.

The largest gap developed after hatching

About 66.9% of pairs in the high-salinity areas produced no young, compared with 34.5% in the fresher areas. A total of 27.1% of high-salinity pairs were not documented laying eggs, versus 3.6% at low salinity. Eggs of known outcome hatched at broadly similar rates in the two groups, but the pattern diverged sharply after hatching: roughly 45–47% of young survived to near fledging at high salinity, compared with 84% at low salinity. High-salinity pairs also lost about 1.1 young on average after hatching, compared with 0.3 at the low-salinity sites. The large reproductive gap therefore accumulated mainly through failure to begin breeding and loss of young rather than through poorer hatching alone.

No high-salinity area reached the population-maintenance level

None of the ten high-salinity study areas reached 1.15 fledged young per pair, a previously estimated level needed for young production to roughly offset mortality and maintain the Chesapeake population over time. Persistently producing fewer young than this means an area functions as a demographic sink unless birds are supplied by immigration. Only two of the ten high-salinity areas reached a lower published threshold of 0.8, while both low-salinity sites exceeded both thresholds. At four high-salinity areas with historical data, reproductive rate fell from 1.57 young per pair in 1985–1988 to 0.46 in 2024, while the proportion of hatched young lost before fledging rose from 13.2% to 50.1%.

Salinity also marks different fish communities

The authors interpret the combination of fewer breeding attempts, smaller clutches, brood reduction and chick losses as evidence of food stress. Salinity is not proposed as a direct physiological cause in the ospreys; it separates parts of the bay with different prey communities. Breeding ospreys in high-salinity waters rely particularly heavily on Atlantic menhaden (Brevoortia tyrannus), an energy-rich schooling fish in the herring family, whereas birds in fresher reaches depend more on fish such as catfish and gizzard shad. The authors identify reduced menhaden availability as the leading explanation, drawing on earlier Chesapeake Bay studies that documented declining menhaden deliveries to nests and improved osprey reproduction when nests were experimentally supplied with the fish.

The 2024 survey did not directly measure adult menhaden abundance, however, and the authors note that there is no fisheries-independent monitoring of adult menhaden inside Chesapeake Bay. The effect of commercial harvest on local availability therefore remains disputed, and other ecological factors may also affect the fish. The study’s direct finding is that osprey reproductive performance was substantially lower across the high-salinity main stem of the bay than at the two low-salinity reference areas.

Sources and image credit

  1. Watts, B. D., Hines, C., Byrd, M. A., Lukei, R. F. Jr., Paxton, B. J., Duval, L., Spiewak, K. E., Kearns, G. D., Day, D. D., & Rattner, B. A. (2026). Widespread reproductive deficits in Chesapeake Bay ospreys. Frontiers in Marine Science, 13, 1685158. https://doi.org/10.3389/fmars.2026.1685158Primary peer-reviewed open-access study published 8 June 2026. It monitored 571 occupied osprey territories at ten high-salinity and two low-salinity Chesapeake Bay sites in 2024 and compared four high-salinity sites with 1980s data. The authors suggest reduced Atlantic menhaden availability as the primary driver but note that adult menhaden abundance was not measured directly.

Image: Chris Williamson / Wikimedia Commons / Flickr — adult osprey (Pandion haliaetus) and young on a nest at Solomons Island, Maryland, USA, photographed 12 July 2009; representative Chesapeake Bay breeding image, not identified study birds · CC BY 2.0 · Source

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