Comparative analysis of weight in male and female sihek ( <i>Todiramphus cinnamominus)</i> chicks at Brookfield Zoo Chicago

Sexual size dimorphism (SSD) commonly occurs in birds, with males typically larger than the females. One notable exception is the family Alcedinidae, which displays moderate reverse SSD. Using body mass data collected from birds hatched at Brookfield Zoo Chicago between 1989-2023, we confirm that the sihek (Guam kingfisher, Todiramphus cinnamominus ) exhibits moderate reverse SSD. Female chicks are significantly larger than males at fledging (t = 4.23, df = 23.33, p = 0.003). Adult birds 50 days and older display significantly different body masses, with females weighing on average 7% more than males (t = 3.20, df = 49.91, p = 0.00238). We assessed the growth patterns of male and female sihek chicks using logistic nonlinear mixed models and found that the null model was a better fit (lower AIC); therefore, there was no significant difference between the growth parameters for each sex from hatch through forty days. These results contribute to the body of work investigating the evolution of SSD in birds and provides data on the growth and development of an extinct-in-the-wild species.