Reversed sexual size dimorphism (RSSD), where females within a given species are larger than their male counterparts, is a phenomena observed across a few avian taxa including hawks and eagles (Accipitriformes), falcons (Falconiformes), waders (Charadriiformes), and owls (Strigiformes). While the mechanisms driving the evolution of this phenomenon are widely discussed, the proximate effects of RSSD on development and juvenile morphology are not well understood. Life history characteristics, such as brood size, influencing avian morphology are also important in understanding long-term patterns in development. I studied development of Flammulated Owls (Otus flammeolus), an RSSD species that tends to raise offspring in broods of 2-3 owlets, to better understand these relationships. I analyzed development using two measures: mass and wing feather length. First I determined the gender of all 2011 nestlings based on genetic analysis of blood samples collected from owlets captured and bled in 2011. Gender data since 2003 were already available. Growth analysis on a total of 189 owlets revealed that females reach a higher asymptotic mass than males. Broods consisting of three owlets reached a higher asymptotic mass than broods consisting of 2 owlets, an unexpected result based on previous research. The differences in maximum juvenile mass in broods of different sizes may be biased due to divergent sex ratios within broods; 57% of owlets in broods of three were female while 38% in broods of two were female. Even so, trends remained the same when males are compared with other males and females with other females in same-sized broods. Wing size differed little between the genders and broods. Juvenile body condition ultimately informs adult viability and fitness, thus it is important to understand these and other selective factors that influence avian development.
Optimal clutch size has been an important focus within evolutionary biology since David Lack’s innovative work in 1947. Prey abundance, typically thought to limit clutch size, may be especially limiting in raptors, since the females contribute minimally to prey provisioning. Studying species with significant energetic constraints may illuminate the relationship between energetics, parental division of labor and clutch size. Flammulated Owls (Psiloscops flammeolus) are a small raptor with prey that is small relative to their body size, further constraining flammulated owls energetically when compared to raptors with larger prey. I hypothesized that female flammulated owls with clutches of three will contribute more to prey deliveries than females with clutches of two, while male prey delivery rates will not vary with brood size. Prey delivery data from 115 flammulated owl nests in the Front Range of Colorado were recorded from 2004-2013. During the second half of the nestling period broods of three received more prey deliveries than broods of two (p < 0.05). Additionally, during the second half of the nestling period no significant difference was found between male and female prey delivery rates for broods of three (p > 0.05). However, among adults with broods of two, males provided significantly more prey than females (p < 0.05). Male prey delivery rate between brood sizes was not significantly different (p > 0.05). These results indicate that broods of three may require greater energy expenditure than broods of two from the female, but not the male parent. While these results pertain to a bird with a small inflexible clutch size, similar research on birds with larger more flexible clutch sizes may reveal how and if clutch size and parental division of labor have co-evolved within avian taxa.