HOW BIRDS RESPOND TO ANTIOXIDANT CAPACITY, OXIDATIVE DAMAGE, AND FUEL STORES DURING MIGRATION

During migration, birds have an elevated metabolic rate and rely heavily on fat for fuel, creating a state where oxidative stress may be high if not counterbalanced by antioxidants. However, no previous studies have examined how birds utilize different aspects of the antioxidant system to combat oxidative damage during flight training, whether dietary antioxidants are absorbed and transported to the site of reactive species generation, the mitochondria, or how oxidative status interacts with fuel stores to shape decisions birds make during migratory stopovers. In this dissertation, I first introduce the physiological challenges of increased production of reactive species confronted by birds during migration, and the interacting roles of antioxidants in protecting birds from oxidative damage (Chapter 1). Next, I examine how a bird’s multifaceted antioxidant system (enzymatic, nonenzymatic, and dietary) responds to increased oxidative demands associated with flight across time (Chapters 2 and 3). Finally, I examine how fat stores and oxidative status influence stopover behaviors in wild songbirds during fall migration. To investigate the acute and chronic effects of an energy-intensive activity (flight) on redox homeostasis (Chapter 2), I drew blood samples from Zebra Finches (Taeniopygia guttata) exposed to an energy-intensive activity (60 min of perch-to-perch flights twice a day) on the first day of flight, after 13 days of training and after 44 days of training. Although well documented in mammals, this is the first study to examine how energy-intensive training affects the antioxidant system and damage by reactive species in flight-trained birds over many weeks. I measured multiple components of the antioxidant system: an enzymatic antioxidant (glutathione peroxidase, GPx) and non-enzymatic antioxidants (measured by the OXY-adsorbent test) as well as a measure of oxidative damage (d-ROMs). At no point during the experiment did oxidative damage change. I discovered that exposure to energy-intensive exercise training did not alter baseline levels of GPx (flight day, F2, 15= 2.42, P = 0.122; training group, F1, 15 = 0.02, P = 0.97; Interaction, F2, 15= 1.63, P = 0.227), but induced exercise-trained birds to maintain a higher non-enzymatic antioxidant status as compared with untrained birds (untrained, F2, 25 = 5.51, P = 0.010; training group, F1, 18 = 0.01, P = 0.893; Interaction, F2, 25 = 4.26, P = 0.026). Novel intense exercise (Day 1) increased the enzymatic but not the non-enzymatic antioxidant system relative to baseline. GPx activity was elevated above baseline in trained birds immediately after completion of the second one-hour flight on each of the three sampling days (flight day, F2, 13 = 0.54, P = 0.593; training group, F1, 15 = 23.17, P = 0.002; Interaction, F2, 13 = 3.05, P = 0.082), and non-enzymatic antioxidants were acutely depleted during flight after 13 and 44 days of training (flight day, F2, 13 =1.27, P = 0.300; training group, F1, 15 = 4.36, P = 0.028; Interaction, F2, 13 = 4.15, P = 0.041). The primary effect of exercise training on the acute response of the antioxidant system to 2-hr flights was a more coordinated response between the enzymatic (GPx) and non-enzymatic components of the antioxidant system of birds and this reduced the oxidative damage associated with exercise. To better understand the bioavailability of dietary antioxidants for birds exposed to the energetic and oxidative demands of flight, I gavaged a subset of Zebra Finches with deuterated α-tocopherol (Chapter 3). I show for the first time that an ingested lipophilic antioxidant, α-tocopherol, reached the mitochondria in the flight muscles of a songbird and that its bioavailability depended on exercise. I also examined the time course over which deuterated α-tocopherol appeared in the blood and mitochondria isolated from pectoral muscle of Zebra Finches exposed to 60 min of perch-to-perch flights two times in a day, or that were relatively sedentary in cages. Deuterated α-tocopherol was found in the blood of Zebra Finches within 6.5 hrs (R2 = 0.83, F5,15 = 19.74, P\u3c0.001) and in isolated mitochondria within 22.5 hrs (R2 = 0.77, F7,10 = 4.77, P = 0.01), but only if the birds were exercise-trained. These results indicate that exercise affected the timecourse and facilitated the absorption and deposition of vitamin E to tissues and organelles. To examine how antioxidants interact with fat stores to influence stopover decisions (how long to stay on stopover and which direction to leave) during fall migration, I conducted a field experiment and manipulated the condition (fat stores and antioxidant capacity) of wild Hermit Thrush, Yellow-rumped Warblers, Red-eyed Vireos, and Blackpoll Warblers on Block Island, Rhode Island (41°130N, 71°330W; Chapter 4). I tested the hypothesis that birds with greater fuel stores and antioxidant capacity have shorter stopovers and depart in a seasonally appropriate direction compared to lean birds with low antioxidant capacity. I used a 2 X 2 factorial experiment (high, ad libitum, or low, maintenance, food availability; dietary anthocyanins or no anthocyanins) in four species of birds that differed in migration strategy: Yellow-rumped Warblers (Myrtle subspecies, Setophaga coronata coronata), Hermit Thrushes (Catharus guttatus), Red-eyed Vireos (Vireo olivaceus), and Blackpoll Warblers (Setophaga striata). Prior to release, I attached digitally coded VHF transmitters to assess stopover duration and direction of departure from the stopover site using the Motus network. Non-enzymatic antioxidant capacity increased during the captive refueling period for Hermit Thrushes, Red-eyed Vireos, and Blackpoll Warblers fed ad lib diets (F1,2 = 6.79, P = 0.01; F1,2 = 10.63, P = 0.03; F1,2 = 13.60, P = 0.004, respectively). Oxidative damage decreased in plasma from Red-eyed Vireos and Hermit Thrushes across all treatment groups during captivity (F1,2 = 1.15; P = 0.31; F1,3 = 1.12, P = 0.30), but increased in the plasma of Blackpoll Warblers fed an ad lib diet (F1,2 = 104.90; P = 0.009). No measure of oxidative capacity changed during captivity in Yellow-rumped Warblers (F1,3 = 3.21, P = 0.89). Stopover duration was shorter for Hermit Thrushes, Red-eyed Vireos and Blackpoll Warblers fed ad lib as compared to maintenance food (F1,3= 7.52, P = 0.01; F1,2= 2.96, P= 0.09; F1,2= 18.01, P= 0.004 respectively), but not for Yellow-rumped Warblers (F= 0.69, P= 0.41). Departure direction was not related to fuel stores or antioxidant condition, and birds from all species primarily reoriented north when departing Block Island. This is the first study to show the importance of antioxidant capacity as a physiological cue for migratory behavior, and the results highlight the influence of fat stores and antioxidant capacity on the timing of migratory stopovers