Behavior

Nest building – Breeders choose between building over, or upon old nests, or removing previously used nesting materials. Often, the male commences tossing small sticks and twigs over old nest materials (and the female follows suit), forming a platform upon which the pair constructs a new nest cup. Alternatively, the nest pair hollows out a depressions in the old nest substrate, which often consists of a compacted mud-like surface composed of discarded seeds, frog, bird, and lizard skeletons, etc., all glued together by the nestlings’ excrement. The pair (most often the female) then constructs open-cup an nest in the excavated hollow. If the "ownership" of a nest site is uncertain, prospecting individuals will slip in and steal nest materials. As an extreme, some pairs actually pitch out old nests in their entirety before constructing new platforms and nest cups, especially if previous nests are elevated to the point that they are close to the entrance hole.

Nest sanitation – As with many passerines, Pearly-eyed Thrasher eats or removes the nestlings’ fecal sacs, discarding them about 10–30 m from their nest boxes. Adult breeders will remove young, usually unfeathered, nestlings that have expired, discarding the corpses 20–30 m from their nest boxes.

Pearly-eyed Thrasher protects the nest site year-round and greatly increases the home range during the nonbreeding season when they can fly hundreds of meters to forage in loose flocks or aggregations with conspecifics in fruiting trees.

Nest-pair Territorial Defense – Within the Luquillo Experimental Forest (LEF) in northeastern Puerto Rico, an experiment was conducted to determine whether pearly-eyes guard their nest sites on a year-round basis (Arendt 2006b: Chapter 8, page 267). Thrasher mounts were placed near nest boxes during the nonbreeding season. Both sexes of the incumbent pairs viciously attacked the mounts, either by diving at them and producing audible wing whirrs (see Nonvocal Sounds), or by pecking at the head, neck, and dorsal areas of the mounted thrasher from nearby perches. Judging from the voracity of the attacks, coupled with several years of observations documenting similar combative behavior among would-be nesters competing for nest boxes (interference competition), especially following devastating storms such as ravaging hurricanes, adequate nest sites most assuredly are limited in a forest in which the annual rainfall exceeds 3,000 mm.

Male territorial defense – Neighboring territorial males vie for vertical dominance by trying to climb higher in the vegetation than their opponents, constantly flashing their white tail spots, jerking their wings, and advertising their white-edged tertials (t7–t9).

In the same montane pearly-eye population in the LEF, between 23 January and 28 November 2003, Beltrán et al. (2010) determined the home range (95% adaptive kernel) and core area (50% adaptive kernel) sizes for 10 radio-tagged males. Significant differences were found among the breeding stages in home range and core area sizes. Home range sizes were significantly larger during the post-breeding period (median = 17.9 ha) than during the breeding period (median = 2.1 ha). This suggests that the increase in home range size during post-breeding season was a response to the decrease in palm fruit, which decreased in abundance during the breeding season.

Courtship and Pair Bonding – Following is an example of pair bonding that took place in palo colorado forest within the Luquillo Experimental Forest in northeastern Puerto Rico in March of 1981 (condensed from Arendt 2006b: Chapter 7, pages 183–184).

A male thrasher alighted at the entrance hole of its nest box carrying a small oval-shaped, green leaf in its bill. He began to turn inward and outward, inward and outward, poking his head inside the box and then withdrawing it, still grasping the leaf. He then flew to a female perched about 3 m from the thrasher box entrance. The male dropped the leaf as it alighted about 10 cm below the female. He then thrust his bill upward to almost a 90º angle in a fashion similar to that of icterids when they are courting females or trying to intimidate rival males. The male then drooped its wings, cocked its tail, and puffed out its body feathers until he appeared much larger than his actual size. The female, however, remained indifferent. The male then mixed chattering (almost a rattle) with soft song. The female, however, continued to ignore the male. She flew up to the mid-story canopy and began feeding from a bromeliad. The male continued to sing at its previous site, but he deflated his body feathers and appeared more relaxed as he sang. Within a couple of minutes, the female returned to her original perch a few centimeters above the male. The male resumed his previous behaviors. His body quivered slightly as he vocalized. He attempted to move very close to the female, but she withdrew and flew to the entrance hole of the nest box, peered inside, and appeared very anxious as if contemplating entering, but then flew off and resumed foraging. After additional behaviors by both sexes similar to those previously described, the female returned to the nest box with strands of nesting material in her beak and began constructing a nest. Eventually, the pair successfully fledged two young.

Cuckoldry – In the case of the Pearly-eyed Thrasher, and contrary to traditional cuckoldry, it may be the males who are the more frequent unfaithful partners of the pair. During a long-term study of a montane population in northeastern Puerto Rico (Arendt 2006b: Chapter 7, pages 184–185), territorial females guarded their nest boxes and did not allow intruders of either sex near them. On the other hand, on several occasions, especially late in the season, males were observed allowing what were assumed to be young, and probably first-time, female breeders not only to enter their breeding territories, but even approach and inspect (enter) their nest boxes. There were several instances in which the last clutches of the season were laid by unbanded females at boxes in which known (e.g., banded) females had laid all previous clutches. And more often than not, it was the banded females that returned to breed again in the same box in subsequent seasons. This lends credence to the possibility that resident males not only permit prospecting females within their territories but probably also inseminate them and allow them to breed in their boxes while maintaining their pair bonds with their previous mates.

Pearly-eyed Thrasher generally is very pugnacious, but especially so when guarding its individual space, nest sites, and breeding territory (Arendt 2006b: Chapter 8, pages 267–269). Its relentless aggression and tenacity immediately impress anyone who has ever observed it in physical combat with conspecifics or other species. Oftentimes, territorial adults are so caught up in the “heat of battle” that they lock claws and beaks and plummet to the forest floor, sometimes from heights of more than 10 m. This behavior often results in the injury or even death of the opponents. Rolle (1965) reported the death of a nesting Red-legged Thrush (Turdus plumbeus) resulting from this form of interference competition when a female pearly-eye nested within 10 m of the thrush. Both individuals may have died consequential to the intense battles that raged for more than 3 days. The thrush was found dead on the ground below her nest. The thrasher, although not located, disappeared, and eggs were never laid in her nest.

Occasionally pearly-eyes fall prey to raptors such as Red-tailed Hawk (Buteo jamaicensis). In 1983, in the Luquillo Experimental Forest, Puerto Rico, Eduardo Santana C. (personal communication) observed thrasher adults and unfeathered nestlings being brought as prey items to feed nestling red-tails. Also, radio-tagged thrasher fledglings have fallen prey to foraging red-tails when they were unable to escape predation as a result of their weakened condition stemming from heavy botfly (Philornis sp.) larval infestations as nestlings.

On the island of Montserrat, W.I., American Kestrel (Falco sparverius) has undergone ecological release, filling the niches of Buteo hawks, often observed soaring high above the canopy, and forest falcons, often seen perch-hunting under the canopy and then chasing their prey, including bats and birds, through the forest in earnest pursuit. In 1984, I saw a large female kestrel attack from behind and carry off an adult pearly-eye, albeit with great difficulty and a visibly labored flight.