This species breeds from about 1,000 m (3,300 ft) altitude (Pacific slope) or 1,500 m (4,900 ft) (Caribbean slope) up to 2,600 m (8,500 ft) and is found in canopy, woodland edge and semi-open habitats such as pasture with some trees. The nest is a thin cup constructed on a bulky twig base 1–3 m (3.3–9.8 ft) up in a small tree or amongst vines. The female lays two brown-spotted pale blue eggs between March and May.
The adult black-thighed grosbeak is 20 cm (7.9 in) long, weighs 70 g (2.5 oz), and has a massive grey bill. The male has a yellow head, rump and underparts, an olive-edged black back, and black wings, thighs and tail. There is a white patch on the flight feathers. The female is paler with more olive on the back and a smaller white wing patch. Immatures are duller and more olive-tinged, and have streaking and mottling on the body plumage.
- Stiles, F. Gary; Skutch, Alexander F. (1989). A Guide to the Birds of Costa Rica. Comstock Publishing Associates. ISBN 0-8014-9600-4.
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- Black-thighed grosbeak videos, photos, and sounds at the Internet Bird Collection
- Black-thighed grosbeak photo gallery at VIREO (Drexel University)
- Black-thighed grosbeak species account at NeotropicalBirds (Cornell University)