Why is the smallest bird so small? High sexual dimorphism in Bee Hummingbird ( Mellisuga helenae )

Evolution of small body size may be influenced at a proximate level by physiology or energetics, and at a more ultimate level by foraging niche (ecology), fecundity selection, or sexual selection for aerial agility. Moreover, sexual size dimorphism (SSD) may arise when selection pressures on males and females differ. Here we assessed hypotheses of why Bee Hummingbird (Mellisuga helenae) is the smallest bird, through field observations in Cuba during the breeding seasons of 2012 and 2015, and by assembling a dataset on egg allometry of 165 hummingbird species. We show that Bee Hummingbird has pronounced SSD: adult males, at 1.74 g (n = 8, measured with a digital scale) are indeed “smallest bird in the world,” while adult females, at 2.22 g (n = 8), are not the smallest female bird. If body size of both sexes is considered jointly, Esmeraldas Woodstar (Chaetocercus berlepschi) may be smaller. Why is SSD so pronounced in the Bee Hummingbird? Courtship displays of seven males did not include behaviors of particularly high agility, thus we did not uncover evidence that aerial agility has driven SSD. We did not find clear evidence supporting the niche differentiation hypothesis: although bill length has subtle dimorphism (female bill: 11.8 ± 0.4 mm; male: 10.6 ± 0.4 mm), it is similar to close relatives that lack SSD. Among 165 hummingbird species, egg size is hypoallometric: small hummingbirds lay relatively large eggs, nearly 20% of the female’s body mass. Bee Hummingbird lays eggs of typical size for its body size, and a few other species lay smaller eggs. Male Bee Hummingbird is the “smallest bird” because males are substantially smaller than females, but the underlying evolutionary causes of such pronounced sexual size dimorphism remain unclear.