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The effects of gender and brood size on the development of Flammulated Owl (Otus flammeolus) nestlings

by Jones, Isabel


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.


Includes bibliographical references.

Administrative Notes


Copyright restrictions apply.
Colorado College
Digital Origin
born digital
44 pages : illustrations (some color)
Senior Thesis -- Colorado College
Thesis Advisor
Linkhart, Brian
Department of Biology
Degree Name
Degree Type
Bachelor of Arts
Degree Grantor
Colorado College
Date Issued