Ferruginous hawks that spend their winters in California have been unexpectedly thriving. Yet, their numbers have been significantly declining in their preferred grassland habitats for over 20 years.
In contrast, many other species of grassland birds are following the projected trend, with their populations going down as more and more of the open areas they inhabit are lost.
In a new paper, a team of researchers is outlining possible explanations for the species’ growth. Ferruginous hawks (Buteo regalis) are difficult to study because they do not show up consistently at migration sites.
Unlike other hawks, they do not concentrate along common topographical features like ridges. As a result, there is a lot of conflicting data on their populations, so their conservation status varies across different regions and countries.
Many of the hawks wintering in California breed in Washington or Oregon, where they are classified as endangered and sensitive-critical.
The research team analyzed citizen science data from the Washington and Oregon wintering grounds to untangle the species’ mixed population trends.
They examined five Bird Conservation Areas in California, where Christmas Bird Counts (CBCs) were carried out from 1998 to 2022. Every December, CBCs are organized by volunteers and bird-related organizations.
Their counts help provide a clearer picture of the hawks’ range than what typical research efforts could achieve.
The team’s analysis revealed that the hawks’ habitat quality has declined significantly over the span of 25 years. But during that same period, the numbers of ferruginous hawks actually increased in winter across California.
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