Paltry Tyrannulet Zimmerius vilissimus
Version: 1.0 — Published December 15, 2017
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Information on the breeding biology of Paltry Tyrannulet is based on Skutch (1960), who detailed the nesting biology of this species in Costa Rica (subspecies parvus). In this region, breeding is initiated in the dry season (January to March), and continues to July and August.
The nest is a vertically-oriented oval shape with an entrance on the side, typically built into drooping moss or lichens below a horizontal limb, and occasionally within dried Cecropia leaves. Skutch observed two nests of Paltry Tyrannulet that were built into the lower part of an inactive nest of Yellow-olive Flycatcher (Tolmomyias sulphurescens). Nests are constructed ca 2-15 meters above the ground in dead or living trees, and apparently are constructed solely by the female. The nest is composed of mosses, fibers, and other vegetation, assembled using spider webs, and lined with soft materials from seeds.
The clutch is two; the eggs are dull white or whitish with rusty speckling (Skutch 1960, Howell and Webb 1995). Eggs are laid in the morning, but well after sunrise (Skutch 1960). Mean egg dimensions are 17.5 x 13.2 mm (n = 7, range 16.7-18.7 mm x 12.7-13.5 mm, subspecies parvus; Skutch 1960)
Skutch (1960, 1997) observed two nests for five to six hours; during that time the females averaged 27.4 and 32.1 minute incubation bouts, and 13.3 and 12.5 minute recesses, respectively. Apparently only the female incubates. Incubation lasts 16-17 days, and the nestlings remain in the nest for 18-20 days. Their mistletoe-dominated diet extends to the nestlings, with parents regurgitating drupes to the nestlings. The nestlings regurgitate the seeds in and around the nest. Nestlings' digestive systems are apparently less efficient processing the drupes, as the seeds that are regurgitated may still have some sticky pericarp remaining, and the parents may eat these regurgitated seeds (Skutch 1997). In Costa Rica, Paltry Tyrannulets may nest twice in a year. Skutch (1997) monitored 39 Paltry Tyrannulet nests in Costa Rica, and 41% successfully produced young.
Elsewhere, Dickey and van Rossem (1938) reported breeding in El Salvador in April (subspecies vilissimus); Howell (1957) collected a male with enlarged testes in Nicaragua in August (subspecies parvus); nest-building is reported from Panama in late February, and nestlings in July (Willis and Eisenmann 1979; subspecies parvus); and specimens in breeding condition are reported from Colombia from March-November (Hilty and Brown 1986; improbus group).