Drivers and Direct Impacts of Lean Mass Dynamics on the Stopover Ecology and Migratory Pace of Nearctic-Neotropical Migrant Songbirds in Spring

Annual migration in songbirds is one of the most demanding life-history stages. It represents a period of high mortality, yet there is still much unknown about the ecological correlates that influence its successful completion. After long non-stop migratory flights, birds require a stopover period to rest and replenish depleted energy reserves. Birds use fat as the primary fuel to power long-distance flights. However, birds also burn lean tissue, which results in significant reductions in muscle and organ masses. The discovery and quantification of lean mass catabolism represented a paradigm shift in migration ecology because non-fat components were thought to remain homeostatic. Because rebuilding protein is slow, muscle and organ breakdown during migration may dramatically prolong stopover periods and delay overall migration time, which in turn dramatically reduces breeding success. Therefore, the breakdown of lean tissue, the conditions that lead to it, and its consequences are important considerations in understanding the migration strategies of birds. Through this dissertation research, I aim to understand the impact of weather on body condition and how physiological condition impacts subsequent migratory performance. I investigate (1) how weather impacts the lean mass of songbirds after crossing an ecological barrier, and (2) how body condition after crossing an ecological barrier affects stopover duration, refueling rate, and habitat use. My predictions are that higher nightly temperatures or drier conditions experienced during migratory flight will correspond with lower lean body mass on arrival; and that birds with lower lean body mass will require longer stopovers, different habitat, or higher foraging effort to continue migration. I used an integrative approach, combining the field and lab, to better understand how weather experienced during flight can impact the body condition of migratory birds and how this can influence the entire migratory cycle. By using Quantitative Magnetic Resonance (QMR) technology in combination with a novel automated radio-telemetry system, my research provides unprecedented access to detailed physiological and movement data for small migratory songbirds. This research underlines that successfully crossing the Gulf of Mexico may be a key driver of physiological and morphological adaptations. My findings challenge the current paradigm that birds with low lean mass require longer stopover and demonstrates that species under time constraints may shorten stopover even when in poor condition, departing in sub-optimal body condition

The Strength of Migratory Connectivity for Birds En Route to Breeding Through the Gulf of Mexico

The strength of migratory connectivity is a measure of the cohesion of populations among phases of the annual cycle, including breeding, migration, and wintering. Many Nearctic‐Neotropical species have strong migratory connectivity between breeding and wintering phases of the annual cycle. It is less clear if this strength persists during migration when multiple endogenous and exogenous factors may decrease the cohesion of populations among routes or through time along the same routes. We sampled three bird species, American redstart Setophaga ruticilla, ovenbird Seiurus aurocapilla, and wood thrush Hylocichla mustelina, during spring migration through the Gulf of Mexico region to test if breeding populations differentiate spatially among migration routes or temporally along the same migration routes and the extent to which within‐population timing is a function of sex, age, and carry‐over from winter habitat, as measured by stable carbon isotope values in claws (δ13C). To make quantitative comparisons of migratory connectivity possible, we developed and used new methodology to estimate the strength of migratory connectivity (MC) from probabilistic origin assignments identified using stable hydrogen isotopes in feathers (δ2H). We found support for spatial differentiation among routes by American redstarts and ovenbirds and temporal differentiation along routes by American redstarts. After controlling for breeding origin, the timing of American redstart migration differed among ages and sexes and ovenbird migration timing was influenced by carry‐over from winter habitat. The strength of migratory connectivity did not differ among the three species, with each showing weak breeding‐to‐spring migration MC relative to prior assessments of breeding‐wintering connectivity. Our work begins to fill an essential gap in methodology and understanding of the extent to which populations remain together during migration, information critical for a full annual cycle perspective on the population dynamics and conservation of migratory animals

Use of a Florida Gulf Coast Barrier Island by Spring Trans-Gulf Migrants and the Projected Effects of Sea Level Rise on Habitat Availability.

Barrier islands on the north coast of the Gulf of Mexico are an internationally important coastal resource. Each spring hundreds of thousands of Nearctic-Neotropical songbirds crossing the Gulf of Mexico during spring migration use these islands because they provide the first landfall for individuals following a trans-Gulf migratory route. The effects of climate change, particularly sea level rise, may negatively impact habitat availability for migrants on barrier islands. Our objectives were (1) to confirm the use of St. George Island, Florida by trans-Gulf migrants and (2) to determine whether forested stopover habitat will be available for migrants on St. George Island following sea level rise. We used avian transect data, geographic information systems, remote sensing, and simulation modelling to investigate the potential effects of three different sea level rise scenarios (0.28 m, 0.82 m, and 2 m) on habitat availability for trans-Gulf migrants. We found considerable use of the island by spring trans-Gulf migrants. Migrants were most abundant in areas with low elevation, high canopy height, and high coverage of forests and scrub/shrub. A substantial percentage of forest (44%) will be lost by 2100 assuming moderate sea level rise (0.82 m). Thus, as sea level rise progresses, less forests will be available for migrants during stopover. Many migratory bird species' populations are declining, and degradation of barrier island stopover habitat may further increase the cost of migration for many individuals. To preserve this coastal resource, conservation and wise management of migratory stopover areas, especially near ecological barriers like the Gulf of Mexico, will be essential as sea levels rise

Data Cohen et al 2018 Ecography

Raw stable isotope values from feathers and toenails of individual migrating birds captured at three sites over three years

Transects on St. George Island, Florida, USA.

<p>Transects were conducted at two study sites: (A) Nick’s Hole and (B) Unit 4. Observers detected migrants by sight and/or sound while walking nine transects: one in Nick’s Hole and eight in Unit 4. Avian abundance was calculated as number of migrants observed divided by transect length in each cell.</p

Environmental Parameters for Sea Level Affecting Marshes Model.

<p>Environmental Parameters for Sea Level Affecting Marshes Model.</p

Most Common Nearctic-Neotropical Migrant Species Detected Visually or Audibly by Observers during Transects.

<p>Most Common Nearctic-Neotropical Migrant Species Detected Visually or Audibly by Observers during Transects.</p

Sea Level Affecting Marshes Model (SLAMM 6.0).

<p>Sea Level Affecting Marshes Model (SLAMM 6.0).</p

Results of Sea Level Affecting Marshes Model (SLAMM 6).

<p>The SLAMM simulation modeling results for 2 m sea level rise scenario predict that 98% of palustrine forested area will be lost between 2009 and 2100.</p