The Bay Wren is an attractive, medium-sized wren with a black crown and nape and a distinct white throat. It has a rich chestnut-colored back and underparts, and a rufous tail with black barring. Several races occur in Panama, with their underparts varying geographically in the extent of white and rufous/chestnut and presence or absence and extent of black barring. The western Caribbean slope race has bright chestnut underparts, and the eastern Panama race, the one we encounter in Darién, is mostly white on its underparts and has extensive black barring on its breast, belly and flanks. Intermediate individuals have mostly rufous underparts with black barring. Sexes are similar in all plumages, and juveniles are generally less brightly-colored than adults, with less contrast between the head and body.
The Bay Wren is found in lowlands and foothills up to 750 meters in elevation. It can be found in secondary growth forest and forest edge, in areas where the vegetation is particularly dense, and often near streams and water courses. It is especially fond of dense patches of Heliconia plants. Bay Wrens usually remain well-hidden in dense vegetation, but are inquisitive and readily emerge from their hiding places to investigate observers. They are a very adaptable species, and tolerate habitat modification. Like other wrens, they eat a variety of invertebrates including insects, spiders, earwigs and cockroach eggs. They feed by gleaning prey from vegetation. Bay Wrens are not gregarious and do not associate with other species or in mixed feeding flocks.
Bay Wrens are very vocal, in fact one of the most vocal of the family, sounding off a loud, ringing series of high-pitched whistles, often repeated several times. Their songs are highly variable and complex. They also give a variety of dry, harsh chatters and stuttering calls. They are highly territorial, and females as well as makes will sing to defend territories. Pairs sing together frequently, engaging in antiphonal duetting, like many Neotropical wrens. Interestingly, duetting by pairs is not used in territorial defense, but rather a result of male behavior. Males sing to attract females, and duetting assesses the female’s quality—if the male finds the female’s song attractive, he will guard her from other males. Females initiate duetting, but rarely sing on their own. Females will also direct songs towards other females, perhaps as a form of aggression.
Bay Wrens are monogamous, and pairs remain together year-round. Young will stay with their parents for up to several months. Breeding season varies depending on the location; in Panama, they breed primarily during the dry season, with nest building observed in November and hatchlings in March. The nest is built by the pair, 1.6 to 5 meters above the ground in the joint of an upright branch, and is generally elbow-shaped with a short entrance tube, made of plant stems, grass, small roots and strips of Heliconia leaves. The nest is further decorated with green moss and vine tendrils, and lined with leaf skeletons. In Panama, variation in nest shape has been observed, with some nests more rounded and larger, an untidy ball of palm and plant fibers, and placed at the ends of branches. Clutch size is 2-3 eggs that are white with reddish-brown speckling.
There are seven recognized subspecies of the Bay Wren, divided into two clades, and at least five occur in Panama’s Caribbean lowlands. The two clades are C. castaneus (Bay Wren) and C. nigricapillus (Black-capped Wren); these clades have sometimes been considered separate species, and their taxonomy is complex. The individuals on Isla Escudo de Veraguas in Bocas del Toro are known to be particularly tame. The Riverside Wren of western Panama and southwestern Costa Rica was formerly considered a subspecies of Bay Wren.
The Bay Wren ranges from Nicaragua to western Ecuador. In Panama, the Bay Wren is common on the entire Caribbean slope and on the Pacific slope from the Canal area eastward.