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

Abstract

Aims: We used physiological data, in conjunction with habitat information, to elucidate the interactions between two hybridizing songbirds within a hybrid zone. Background: 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. Objective: 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. Methods: 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. Results: 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. Conclusion: 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.

Latitudinal but not elevational variation in blood glucose level is linked to life history across passerine birds

“…This pattern suggests that baseline glucose is the primary trait affected. The elevated G 0 during periods of heavy rainfall may result from the prolonged stress response, increased activity during short rainless parts of the day, or hyperphagia induced by increased unpredictability of food availability (Fokidis et al, 2012;Vaughn et al, 2020). In addition, a considerable proportion of variation in blood glucose may result from variables that cannot be controlled in the field, such as the time from the last meal (del Rio et al, 1988).…”

“…This pattern suggests that baseline glucose is the primary trait affected by precipitation. The elevated G0 during periods of heavy rainfall may result from the prolonged stress response, increased activity during short rainless parts of the day, or hyperphagia induced by increased unpredictability of food availability (Fokidis et al 2012;Vaughn et al 2020).…”

“…To control for the effect of rainfall (Vaughn et al 2020), we prepared a binary variable with the low level including the observations from the temperate zone and the tropical dry season (average monthly precipitation below 92 mm), and the high level including the observations from the tropical rainy season (average monthly precipitation over 232 mm; climateknowledgeportal.worldbank.org; retrieved July 7, 2021). Species clutch size was obtained from Myhrvold et al (2015).…”

“…Our models controlled for circadian and stress‐induced flexibility (Lobban et al, 2010) and several other potential confounding effects. We expected glucose levels to be higher: in tropical rainy season due to stress associated with heavy rainfall (Vaughn et al, 2020); in females, at least during the breeding season (sex × breeding season interaction), fuelling egg production and incubation (Tomasek et al, 2019); and in species with high proportion of nectar and fruits in their diet (Beuchat & Chong, 1998). Moreover, given that long‐distance flight is primarily fuelled by free fatty acids rather than glucose (Jenni & Jenni‐Eiermann, 1998), a shift in fuel use associated with lower baseline blood glucose levels may have evolved in migratory species (Tomasek et al, 2019).…”

Sources of variation in baseline and stress-induced blood glucose levels in two free-living tropical passerine species

“…Heavy rainfall is an important cue for tropical bird phenology (Quispe et al 2018). Generally speaking, heavy rains are likely to be associated with increased stress and elevated energy metabolism (Vaughn et al 2020, Tomášek et al 2022), which may alter the physiological status of a bird. Thus, as stated by Stutchbury & Morton (2008), tropical birds offer unique opportunities to test ecological and evolutionary theory as their life history traits are diverse and differ from temperate zone models upon which most empirical studies are based.…”