Nesting biology, growth and survival of the Slender-Billed Inezia: Insights into the life history of the South America’s smallest passerine

The Slender-bill Inezia ( Inezia tenuirostris ) is notable as the lightest passerine bird in South America. This species is endemic to the lowlands of northwestern Venezuela and northeastern Colombia, where it inhabits seasonal dry and hot environments. Until now, little was known about its reproductive biology. In this study, we described and evaluate key aspects of the nesting biology of I. tenuirostris . To achieve this, we monitored 43 nests located in forest patches within a very dry forest in an early successional stage. The nests were tiny “ventilated” cups constructed from fine woody stems. Clutch size consisted of two immaculate white eggs, which were incubated for an average of 12.5 days. At hatching, nestlings had black skin with sparse whitish down on the dorsal area and fledged after 11.6 days. The growth rate of the nestlings K was 0.42, with the maximum growth rate occurring after 3.73 days post-hatching, reaching 0.49 grams/day. Survival analysis revealed a sharp decline in nest survival during incubation (a 47% reduction) and a steady decline during chick-rearing (a 23% reduction), resulting in an overall low nesting survival rate of 30%. Our findings suggest potentially intriguing adaptations in I. tenuirostris for surviving in harsh environments, including: (1) the ventilated cup-shaped nest, which likely reduces water absorption and/or mitigates overheating; (2) the black skin of nestlings, which may provide photoprotection against harmful UV radiation; and (3) the accelerated growth rate of nestlings is potentially an evolutionary response to the persistent risk of nest predation.