Breeding

Adelaide’s Warblers are typically reported as breeding from March to June on both Puerto Rico (Bowdish 1903, Wetmore 1916a, Spaulding 1937) and Vieques (Wetmore 1927). However, the timing appears to depend on rainfall, and breeding can begin as early as January in southwest Puerto Rico (Staicer 1996b, pers. obs.) and continue through July (Staicer 1991, Nakamura 1995). Fledglings have been reported from mid-June and early July (Bowdish 1903, Wetmore 1916a), but can occur as early as late March in years with early rains (pers. obs.). In dry ecosystems, nesting may begin earlier in wetter areas (Staicer 1991).  Staicer (1996b) found that nesting was not synchronized among neighboring Adelaide’s Warbler pairs.  Double-brooding may occur if early nests are successful, with initiation of second nest usually, but not always, starting after young disperse (Staicer 1991).

Nests typically are placed in a dense shrub or tree, in a well-concealed location, often in the fork of a branch or twig (Wetmore 1916a, Beatty 1931, Spaulding 1937). However, nests may also be placed in crotches of cacti (Danforth 1926, Beatty 1931), or in bromeliads (Staicer 1991). Nest height ranges from 0.15 m to 6 m high (Raffaele 1989), though heights of 1-3 m seem more typical (Wetmore 1916a, Danforth 1926, Beatty 1931, Spaulding 1937). Nests are cup-shaped, constructed of densely woven grass that is more loosely woven at the base (Spaulding 1937), sometimes mixed with cotton (Beatty 1931). Nests are lined with feathers, hairs and seed stalks of grasses (Wetmore 1916a, Beatty 1931, Spaulding 1937), and may sometimes be externally covered with moss (Wetmore 1916a) or bromeliads (pers. obs.). Outer dimensions are 5.3-6.4 cm in diameter and 5.1-5.7 cm deep; inner dimensions are 3.8-4.5 cm in diameter and 3.6-3.8 cm deep (Beatty 1931, Spaulding 1937).

Adelaide's Warblers lay two to four white or greenish-white eggs, flecked with reddish-brown or chocolate spots (larger and more numerous at the large end of the egg; Beatty 1931, Spaulding 1937, Bond 1971). Eggs measure 14.5-16.3 mm by 11-12 mm (Beatty 1931, Spaulding 1937). Only the female is known to incubate (Spaulding 1937). The male did not sing near the nest during incubation. Adelaide’s Warblers became stealthier about approaching and leaving the nest as incubation progressed, increasing their visual checks; they typically dropped down when departing the nest, although they sometimes flew straight out (Spaulding 1937). Incubation lasts approximately 15 days (A. Wiewel, pers. comm.); about 50% of the day is spent incubating, with incubation periods averaging about 26 min (20-36 min; Spaulding 1937). A female was once seen standing and shading the eggs with spread wings, during a period when she seemed to be heat stressed.

Both sexes brood young, although the male may incubate for shorter periods of time than the female (Spaulding 1937). On the first day, brooding periods averaged 8.8 min (3-17 min) and young were fed every 17.9 min on average; on the second day, brooding periods averaged 10.2 min (3-15 min) and young were fed every 8.6 min (2-35 min; Spaulding 1937). Young appeared to be fed only insects, with the size of prey increasing on the second day. Once, a brooding parent was fed by its mate (Spaulding 1937). Initially, fecal sacs were eaten; later, they were carried away.  Chicks fledge at approximately 10 days of age (A. Wiewel, pers. comm.).

Adults care for fledglings through July and August (Staicer 1991).  Fledglings require at least two months to develop foraging skills, gain experience in social interactions and disperse to become a territory owner or floater (Staicer 1991).

Reproductive success of 12.5% (1 of 8 nests; A. Wiewel, unpub. data) to 38.7% (12 of 31 nests; Nakamura 1995) has been reported. Nest failure in Guánica Dry Forest was attributed to depredation, with at least one nest depredated by Pearly-eyed Thrashers (Margarops fuscatus) (A. Wiewel, pers. comm.).  Nest failure in a savanna-like ecosystem was caused by mongooses (Herpestes auropunctatus), rats (Rattus rattus) and a severe rain storm (Staicer 1991).

Shiny Cowbirds (Molothrus bonariensis) arrived in Puerto Rico before 1955 (Cruz et al. 1989). Adelaide’s Warblers seem to have relatively poor defenses against Shiny Cowbirds; 0 of 7 pairs rejected eggs in experimentally parasitized nests (Post et al. 1990, Nakamura 1995) and aggressive responses to a Shiny Cowbird decoy were low (although 2 of 5 pairs increased their aggressive response over time; Nakamura 1995). However, Adelaide’s Warblers are only rarely targeted by Shiny Cowbirds, even though they occasionally visit their nests; only 3 of 74 nests (4%) were parasitized with single cowbird eggs (Post et al. 1990, Nakamura 1995, Nakamura and Cruz 2000). One of the nests had an egg punctured, presumably by Shiny Cowbirds, and was abandoned (Nakamura and Cruz 2000). However, parents were able to successfully raise Adelaide’s Warbler fledgling(s) in one nest (Nakamura 1995; nest success was not reported in Post et al. 1990). Thus, nest parasitism by Shiny Cowbirds does not seem to be a significant problem for Adelaide’s Warblers.