Cassin's Vireo

Vireo cassinii

Cassin's vireo is a small North American songbird, ranging from southern British Columbia in Canada through the western coastal states of the United States, including California, Oregon, and Washington. The Cassin's Vireo moves down Western United States forests in its migration. During its migration in early spring and late fall, it spends the winter in regions from southern Arizona to southern Mexico, including parts of Baja California and the Pacific coast of Mexico. Source

Where it is now

In the last 8 weeks this bird was recorded 1,300 times across 16 areas (each area is a 5 degree square).

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Movement and weather

Each season it is first heard regularly where daytime highs are around 18°C (typically 16 to 20°C, from 10 areas). That holds across 25° of latitude, which suggests its timing follows temperature more than the calendar.

Over the weeks it was present in the past year, its centre in North America ranged from 41°N in April to 50°N in June, about 1,000 km apart.

Arrival dates

The week it was first heard regularly in each region, after at least six weeks away. Dates differ from south to north, so a region has a range.

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Figures are effort corrected and come from automatic sound identification, which makes mistakes. How this is measured and how it compares with human observers.