Skip to content

Western Marsh Harriers Laid Earlier in Wetter Februaries, and Pairs With Paler Males Bred More Successfully

Adult male western marsh harrier in flight against a clear sky
Research · Reproduction & development

An eight-year study of western marsh harriers (Circus aeruginosus) in Spain’s Ebro Valley found that breeding timing varied with both early-season rainfall and male plumage colouration. Across 219 breeding attempts from 2001 to 2008, pairs laid earlier when February was wetter and when the male had paler plumage.

The rainfall pattern fits the ecology of a raptor breeding in a semi-arid Mediterranean landscape. The authors suggest that wetter conditions early in the season may improve reed growth and other nesting conditions while also increasing primary productivity and prey availability. By contrast, the degree of agricultural transformation around the wetlands showed no clear relationship with laying date in this study.

Paler males were linked to earlier and more successful breeding

Pairs with paler males laid earlier and were also more likely to produce at least one fledgling. Male colouration was used as an indirect indicator of differences in age and behaviour: western marsh harrier males tend to become paler with age, and paler birds have also been associated with bolder behaviour. The study therefore does not show that pale plumage itself improves breeding success.

Late breeding carried a greater cost in wetter conditions

Breeding success generally declined as laying was delayed from the earliest dates, reached its lowest level at intermediate dates and rose slightly among the relatively few very late breeders. Rainfall changed the strength of this pattern: the drop in breeding success with later laying was more pronounced when February precipitation was higher.

Predicted western marsh harrier breeding success by laying date and February precipitation
Predicted breeding success in relation to laying date at different February precipitation levels. Figure 4 from Cacciarelli, Mañosa & Cardador (2026), Journal of Ornithology, CC BY 4.0.

The mechanism behind this interaction is uncertain. The authors suggest that in rainier springs, late breeders may be more likely to miss the period when food availability best matches the demands of growing young. Among pairs that did breed successfully, however, the number of fledglings showed no clear relationship with laying date, climate, male plumage or agricultural transformation.

Because the birds were not individually marked, the researchers could not determine whether the same birds shifted their laying dates between wet and dry years. The results therefore reveal population-level associations between climate, breeding timing and male traits rather than repeated responses measured in known individuals.

Sources and image credit

  1. Cacciarelli, F., Mañosa, S., & Cardador, L. (2026). Laying date and breeding performance of a generalist raptor in agricultural systems is affected by plumage colouration and local climate. Journal of Ornithology.Primary peer-reviewed open-access article. Published and version of record 29 January 2026. CC BY 4.0.

Image: Stephan Sprinz / Wikimedia Commons — adult male western marsh harrier (Circus aeruginosus) in hunting flight on Spiekeroog, Germany, 12 April 2023; representative adult-male image, not one of the birds from the Ebro Valley breeding study. · CC BY 4.0 — https://creativecommons.org/licenses/by/4.0/ · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

Reproduction & development

SPECIES IN THIS STORY

Species in this story

Western Marsh Harrier Circus aeruginosus Explore species

Independent research and conservation news archive

Did you find this information useful?

Wildlife Vagabond is independently built and maintained. Voluntary support helps cover source verification, hosting and continued work on research and conservation news.

Support Wildlife Vagabond

The news archive will remain freely available.