The effects of fluctuating embryonic incubation temperatures and nighttime light exposure on the physiology of zebra finches (Taeniopygia guttata)

Abstract

Anthropogenic effects, such as fluctuating incubation temperatures and light pollution, can have both behavioral and physiological impacts on an organism, particularly within avian species. Specifically, when parents leave the nest to forage, eggs are susceptible to temperature fluctuations, which may have downstream effects on physiology later in life. Additionally, light pollution is also of major concern regarding increased urbanization, which has been found to disrupt circadian rhythms and increase collisions with artificially lit structures in birds. However, little is known about the influences these environmental stressors have on avian species long-term. To address this gap in the literature, we utilized zebra finches (Taeniopygia guttata) as a model to understand the impacts of these environmental stressors. In Chapter 2, we exposed zebra finch eggs to one of three incubation regimes: (1) a Control incubator, which held a constant temperature of 37.4°C, (2) a Low incubator, which held a constant temperature of 36.4°C, or (3) a Periodic incubator, which exposed eggs to five periodic cooling events daily with an average temperature of the Low incubator. Beak photographs were taken at 45-, 60-, 75- and 95-days post-hatch (DPH) to examine the long-term effects of fluctuating incubation temperatures on the development of beak color. Additionally, we exposed these individuals to a repeated capture and restraint protocol to examine the short-term effects of handling on beak color maintenance. Overall, we found that there are age- and sex-specific effects of incubation treatment on beak hue, where females from periodically cooled eggs had decreased beak hues (redder) in adulthood. We also found that eggs laid later in a clutch had decreased beak saturation levels throughout life, regardless of incubation treatment. Females were found with lower beak hues and saturation following a capture and restraint stressor, while males showed increased beak saturation. Males that were subjected to the Low incubation treatment exhibited higher activity levels than those in the Control group. These findings suggest that fluctuating incubation temperatures combined with repeated, short-term stressors can have significant sex-specific effects on sexual ornamentation and behavior. In Chapter 3, we exposed female zebra finches to constant light at night, or nighttime light exposure (NLE) for 23 days to examine the impacts of light pollution on body mass, beak coloration, bacterial killing ability against Escherichia coli, and gut microbial communities. We did not find a main effect of NLE (~160 lux) on body mass, beak color, or bacterial killing ability against E. coli. However, we found that light-treated individuals had significantly lower body masses than control birds at the conclusion of the experiment. Additionally, we found that individuals on the third day of the experiment had higher beak hues (less red) than at the start of the experiment. Birds exposed to constant light tended toward higher bacterial killing abilities on both the third and twenty-third day of the experiment. These findings suggest that the timing and length of NLE play a major role in the physiological effects of an organism, particularly in avian species.