No abstract

Migratory birds use protein as a fuel source during flight, but the mechanisms and benefits of protein catabolism during migration are poorly understood. The tissue-specific turnover rate hypothesis proposes that lean mass loss depends solely on the constitutive rate of protein degradation for a given tissue, and is therefore independent of metabolic rate or environmental stimuli. However, it has been demonstrated that environmental stressors such as humidity affect the rate of lean mass catabolism during flight, a finding that seemingly contradicts the tissue-specific turnover rate hypothesis. In order to resolve this, we placed migratory Swainson's thrushes in either high (HEWL) or low (LEWL) evaporative water loss conditions at rest and while undergoing simulated migratory flight at 8 m s −1 in a wind tunnel to test the impact of both environmental stressors and metabolic rate on the rate of protein breakdown. The total quantity and rate of lean mass loss was not different between flight and rest birds, but was affected by humidity condition, with HEWL losing significantly more lean mass. These results show that the rate of protein breakdown in migratory birds is independent of metabolic rate, but it can be augmented in response to environmental stressors.

A major uncertainty in automated radio‐telemetry studies of small birds is the detection range of receiving antennas. We compared simultaneous daytime detections (± 30 s) by automated and manual radio‐telemetry to assess detection probability and the proportion of transmissions detected for birds on migratory stopover as a function of distance, foraging guild (Black‐throated Blue Warblers, Setophaga caerulescens, and Yellow‐rumped Warblers, Dendroica coronata coronata, represented mid‐canopy foliage gleaners and White‐throated Sparrows, Zonotrichia albicollis, represented a ground forager), habitat type, meteorological variables, tower antenna number (1–4), and the position of a bird relative to the receiving antenna's bearing (offset angle). Our study was conducted at a migratory stopover site in southern Ontario, Canada. Most detections were in dense to sparse forest, and all individuals were within 1.03 km of the automated receiving station. Daily detection probability was near 100% for both foraging guilds. However, within 30 s before and after a manual radio‐telemetry location was made, detection probability and the proportion of transmissions detected by automated radio‐telemetry declined with distance, was higher for warblers than sparrows, and was lowest for 90° offset angles. Our results suggest that when research goals do not require detections with high temporal frequency, e.g., estimation of departure date or daily departure probability, our study design had an effective detection range of at least 1 km. However, where temporal precision is required, e.g., to investigate movements and changes in activity levels during stopover, detection range was ~300 m for ground‐foraging sparrows and 600 m for mid‐canopy foraging warblers, which is much lower than the presumed detection range of antennas under optimal conditions (15 km). This corresponds to a spatial area of coverage for forest‐dwelling birds of ~0.3–1.1 km2. Our results suggest that to optimally configure an automated radio‐telemetry array at the regional scale, investigators should carefully consider detection range and its underlying covariates, including species type, the habitat matrix, and the orientation of antennas relative to preferred habitat.

Cost of flight at various speeds is a crucial determinant of flight behavior in birds. Aerodynamic models, predicting that mechanical power (Pmech) varies with flight speed in a U-shaped manner, have been used together with an energy conversion factor (efficiency) to estimate metabolic power (Pmet). Despite few empirical studies, efficiency has been assumed constant across flight speeds at 23%. Ideally, efficiency should be estimated from measurements of both Pmech and Pmet in un-instrumented flight. Until recently, progress has been hampered by methodological constraints. The main aim of this study was to evaluate recently developed techniques and estimate flight efficiency across flight speeds. We used the 13C-labeled sodium bicarbonate method (NaBi) and Particle Image Velocimetry (PIV) to measure Pmet and Pmech in blackcaps flying in a wind tunnel. We also cross validated measurements made by NaBi with Quantitative Magnetic Resonance (QMR) body composition analysis in yellow-rumped warblers. We found that Pmet estimated by Nabi was ∼12% lower than corresponding values estimated by QMR. Pmet varied in a U-shaped manner across flight speeds in blackcaps, but the pattern was not statistically significant. Pmech could only be reliably measured for two intermediate speeds and estimated efficiency ranged between 14 and 22% (combining the two speeds for raw and weight/lift specific power, with and without correction for the ∼12% difference between NaBi and QMR) were close to the currently used default value. We conclude that NaBi and PIV are viable techniques, allowing researchers to address some of the outstanding questions regarding bird flight energetics.

Most seasonally migrating songbirds exhibit protandry, whereby males arrive to breeding sites in the spring before females. The proximate behavioral mechanisms of protandry are largely unknown for most species, but could include earlier migratory departure from wintering sites by males or overall faster migration by males. Using onset and intensity of migratory restlessness as proxies for departure timing and rate of migration, respectively, we evaluated these 2 hypothesized mechanisms in a Nearctic–Neotropical migrating songbird, the Black-throated Blue Warbler (Setophaga caerulescens). Birds were captured during fall migration, held in captivity over winter, and photostimulated in the spring to induce migratory behavior. Video analysis was used to separately quantify stereotypical nocturnal wing whirring and jumping migratory restlessness behaviors. The birds were then radio-tagged and released in mid-May to compare stopover duration between the sexes and validate migratory restlessness in captivity as a proxy for the motivation to migrate in the field. In captivity males initiated migratory restlessness earlier in the spring than females, demonstrating innate differences in the onset of spring migration in this species. Males also displayed higher-intensity wing whirring behavior, suggesting potential sex differences in flight behavior that could influence migration rate. We found no sex differences in stopover duration in the field following release. However, stopover duration was negatively correlated with total migratory restlessness intensity on the last night the birds were held in captivity, which supports migratory restlessness as a proxy for the motivation to migrate at the individual level.