Glucose Concentrations in Closely Related Titmice (Baeolophus) Species Linked to Regional Habitat Differences Across an Avian Hybrid Zone

Jennifer C. Vaughn1, *, Gary Voelker1, J. Jill Heatley2
1 Department of Wildlife and Fisheries Sciences and Texas A&M Biodiversity and Research Collections, Texas A&M University, College Station, TX, USA
2 Department of Small Animal Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, College Station, TX, USA

© 2020 Vaughn et al.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the Department of Wildlife and Fisheries Sciences and Texas A&M Biodiversity and Research Collections, Texas A&M University, College Station, TX, USA; Tel: +1 214-908-3808; E-mail:



We used physiological data, in conjunction with habitat information, to elucidate the interactions between two hybridizing songbirds within a hybrid zone.


Hybrid zones are ideal regions to examine a variety of ecological, behavior, and evolutionary processes. In addition to genetics, behavior, and morphology, physiological differences may impact hybrid fitness, genetic introgression, and even the stability of a hybrid zone.


To assess physiological differences in hybridizing species, we investigated selected venous blood analytes in two species of songbirds hybridizing along the Balcones Escarpment in central Texas.


Using a portable blood analyzer, we assayed blood samples from Black-crested Titmouse (Baeolophus atricristatus) and Tufted Titmouse (B. bicolor) individuals along a longitudinal transect that included the contact zone. Ecologically, this transect varies from higher elevation semi-arid regions on the Balcones Escarpment (and west across the Edwards Plateau) to lower elevation mesic forests east of the escarpment.


As expected, several blood analytes differed with age, sex, and sedative administration; however, we observed relatively increased blood glucose concentrations in Black-crested Titmice, which occupy the semi-arid habitats of west Texas. Furthermore, glucose concentrations were further elevated following rainfall events. Blood glucose concentrations often increase during stressful conditions and or related to changes in diet.


We suspect that Black-crested Titmice have relatively increased blood glucose concentrations as a product of living in a semi-arid environment that causes chronic stress from unpredictable food and water resources. The link between rainfall and glucose may be a result of the increased and greater diversity of food availability after rainfall. Although further research is needed, we suspect that habitat differences and associated lack of physiological adaptations may be a limiting factor in westward range expansion in the more aggressive Tufted Titmice.

Keywords: Blood gas, Passeriformes, Speciation, Habitat, Geography, Electrolytes, Biochemistry.