Adelaide's Warbler Setophaga adelaidae
Version: 1.0 — Published May 28, 2010
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Adelaide’s Warblers seem constantly on the move while foraging; "quick and active in habits, they feed rapidly through the tips of the twigs, tumbling down through the limbs after a moth or flying out to capture an escaping insect on the wing" (Wetmore 1916a). Staicer (1992) estimated their rate of movement as 0.07 m/sec. This movement can make them difficult to spot, particularly in dense vegetation, but they do not seem to avoid humans. However, Danforth (1937) found them to be much shier on Vieques than on Puerto Rico.
Adelaide's Warblers use a stereotyped display for close-range territory defense, the chitburst display (Staicer 1991). In this display, individuals of either sex show a wide stance, have an upright posture with hunched back, point their tail down, bow their wings out and down, sleek their plumage, gape their bill, and give chitburst calls (see Vocalizations for description of call). Males do not sing immediately prior to or during the display, but females sometimes chip prior to the display (Staicer 1991). In a less aggressive posture, the wings are pressed down slightly and the tail is cocked upright (and sometimes twitched), and individuals chip (Staicer 1991). The wings may also be flicked outwards and down (pers. obs.).
Individuals display at territory boundaries daily (Staicer 1991). Encounters with neighbors often result in chitburst displays or chasing, with most chases appearing to occur between individuals of the same sex (Staicer 1991, pers. obs.). Although individuals will attack neighbors of either sex, males do not attack female floaters (Staicer 1991).
Fights usually occurred between territorial males and male floaters or neighboring males with small territories (Staicer 1991). "During fights, males grasped at one another with their feet and stabbed at each other with their bills, often falling entangled to the ground or becoming caught in the twigs or thorns of the vegetation. Prolonged fighting sometimes resulted in injuries or territory take-overs or both" (Staicer 1991). Persistent attacks on one male often resulted in the attacker taking over his territory (Staicer 1991).
Little is known about their self maintenance activities. In Guánica Dry Forest they bathe by vigorously fluttering their wings, crouching onto the upper-surface or jumping into the undersides of wet leaves (typically in the canopies of saplings or small trees), wetting their feathers thoroughly before preening. Individuals may even make extra-territorial forays to use a particularly suitable tree (pers. obs.). They spend a much greater proportion of their time preening and scratching when they are molting. The head is scratched by stretching the leg over a drooped wing (pers. obs.). They frequently wipe their bill from base to tip on branches when foraging (pers. obs.). Paired males and females typically roost apart (Staicer 1991).
Pairs defend territory year-round (Staicer 1996b). Females defend territories except when breeding (Staicer 1991). In Guánica Dry Forest, territories average 0.42 ha, although it varies throughout the forest (J. Toms, unpub. data). Population density in Maricao was estimated at 1.2/ha, though it is not known whether the habitat was saturated (Delannoy and Cruz 1999). Territories overlap by several meters, and chitburst displays are most common in these areas of overlap (Staicer 1991).
In a savanna-like open forest (Cabo Rojo Wildlife Refuge, southwest Puerto Rico), 20-40% of the population were non-territorial “floaters” (Staicer 1996b). Floaters typically ranged over an area equivalent to about five territories (Staicer 1991). Territorial males tolerated and sometimes associated with female floaters, but only if their mate was not nearby (Staicer 1991).
Female floaters found territories more quickly than males, who often required more than a year to obtain a territory. Juveniles born early in the breeding season have a better chance of obtaining a territory in their first year (Staicer 1991). Males on higher-quality territories attracted mates more quickly (within two days) than those on lower-quality territories (often required weeks or months; Staicer 1991).
Adelaide's Warblers can be captured using playback of songs; however, they can rarely be recaptured using the same technique, and often lure away unbanded mates (Staicer 1991).
Once formed, pair bonds are maintained year-round until one individual disappears (Staicer 1996b). New pairs can form at any time of year, when an unpaired male acquires a new territory or is widowed. Most males pair with “floater” females that are less than 1 year old, but sometimes pair with a widowed neighbor (Staicer 1996b). Individuals are more likely to pair with a neighbor than with a "stranger" (Staicer 1991). Males sometimes pair with females with young, and will feed her fledglings (Staicer 1991). During pair bond formation, males sang and females called frequently, but the vocalizations were not coordinated; the individuals stayed in relatively close proximity (less than a few meters apart; Staicer 1996b). In contrast, established pairs vocalized less frequently, but females responded to their mate’s song with one or more calls; they often became separated while foraging, but maintained vocal contact (Staicer 1996b).
Sexual chases occur during initial pair bond development, and before nesting in established pairs. These chases occurred over short distances (a few meters), with one individual chasing the other closely through and around low shrubs (Staicer 1991). One or both individuals twittered during the chase.
Copulatory behavior was seen only in established pairs (Staicer 1996b). Immediately before copulation, males start singing A songs much faster than usual, and give a “puff” display that causes them to appear rounder and larger (Staicer 1991). The female responds with a pre-copulatory display: she fluffs her body feathers, crouches while stretching her neck and tail upwards, lowers her wings and vibrates them, then throws her head back, opens her bill, and gives a faint high-pitched trill or twitter (Staicer 1991). The male then flies down onto her back and copulates, while singing a copulation song.
Adelaide's Warblers appear to be strictly socially monogamous (Staicer 1991), although no studies have determined whether extra-pair copulations may occur. However, two short-term instances of polygamy/polyandry have been observed: one male was seen with two females for one month, and one female pair-bonded with two males for at least three months, before settling with one of them (Staicer 1991). There are two documented instances of females "divorcing" to pair with a male on a better-quality territory (Staicer 1991).
Social and interspecific behavior
In savanna-like habitat, non-territorial female Adelaide’s Warblers sometimes forage in association with other females, or with migrant warblers (Staicer 1991). In Guánica Dry Forest, Adelaide’s Warblers typically forage singly or with their mate (pers. obs.), but are sometimes loosely associated with Puerto Rican Vireos (Vireo latimeri; Willis 1973). In other areas of the island, they may join mixed-species flocks with Puerto Rican Todies (Todus mexicanus), Lesser Antillean Pewees (Contopus latirostris), Puerto Rican Vireos, Black-whiskered Vireos (Vireo altiloquus), and Elfin-woods Warblers (D. angelae) (Curson et al. 1984).
Adelaide’s Warblers can be aggressive towards other species. They attacked Northern Parulas (Parula americana) and Prairie Warblers (D. discolor) in savanna-like habitat; attacks on Prairie Warblers were particularly abundant during the first two weeks after their arrival (Staicer 1992). Even though Adelaide’s Warblers were smaller than the other species, they were rarely attacked. Males were more commonly involved in aggressive interactions (Staicer 1992). Chitburst displays were not used against other species (Staicer 1991). In Guánica Dry Forest, Adelaide’s Warblers attack American Redstarts (Setophaga ruticilla) and appear to be both dominant and competitively superior (J. Toms, unpub. data).
Pearly-eyed Thrashers (Margarops fuscatus) are known to depredate nests (A. Wiewel, pers. comm.). However, mongooses (Herpestes auropunctatus) and roof rats (Rattus rattus) were the main nest predators in a savanna-like ecosystem (Staicer 1991). A pair of Adelaide’s Warblers, accompanied by a fledgling, were seen aggressively scolding a Puerto Rican racer (Alsophis portoricensis) that was approximately 2 m high in a stand of saplings and within 1-2 m of the fledgling (which was approximately 2.2 m high; pers. obs.). The juvenile was quite young, and may possibly have force-fledged in response to the threat of the snake.
A male Adelaide's Warbler was attacked by an unknown falcon, probably a Merlin (Falco columbarius), in Guánica Dry Forest (pers. obs.). He was singing in the upper canopy of a leafless tree when attacked, emitted an alarm call, and dove towards the ground. Adelaide's Warblers use distraction displays (noise and fluttering wings) when humans approach their nest (Spaulding 1937). They also join in mixed-species mobbing of Puerto Rican Screech-Owls (Megascops nudipes; pers. obs.).