Retreat, detour or advance?
Submitted by editor on 31 March 2021.
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"The ability for migrating birds to stop en route and locate habitat where they can quickly transition from feeders to flyers is critical for a successful migration. In our study, most migrants (75%) retreated inland or detoured around rather than advanced across the Gulf, but this depended on bird species and fat‐based energy stores. Most individuals in lean condition or of smaller bodied species tended to retreat or detour, rather than advance, when departing from the coast."
SPECULATIONS:
"We suspect the feeder/flyer dichotomy influenced behaviors we did not measure and was influenced by weather conditions and migration timing:
•We hypothesize that the individuals in this study also displayed differences in foraging behavior, daytime sleep and general activity consistent with the feeder/flyer dichotomy. We predict that flyers foraged less and were risk averse if foraging, slept more during the day, and were less active (i.e. sit and wait to advance) compared to feeders.
•Prevailing weather conditions are known to influence departure decisions, including in our system, and likely adds a consequential circumstance to the feeder/flyer dichotomy. While individuals advancing from the interior site generally flew into headwinds upon departure, we suspect longer stopover durations at the interior site could also be due to some individuals attempting to await favorable weather conditions – delaying the transition from feeder to flyer.
•A migrant’s spatiotemporal context can influence decisions (e.g. a late bird minimizing time) and circumstances (e.g. more favorable weather later in the season). We hypothesize that we would see less retreating movements and other behaviors associated with feeders later in the season compared to earlier in the season as migrants may need to minimize time as en route resources diminish."
Figure 2. (A) Departure directions of radio-tagged individuals by species from the edge site on Fort Morgan, AL. Departures are categorized as detouring (east–west), retreating (north) or advancing (south). Species include ruby-throated hummingbird (n=52), indigo bunting (n=34), red-eyed vireo (n=145), Swainson’s thrush (n=161) and wood thrush (n=50). (B) Circular histograms of departure directions per species, binned to five degrees. Some individuals of each species had departure directions that were not resolved (i.e. range of possible directions within each category: detouring, retreating or advancing) and were not included in this figure. Species include: ruby-throated hummingbird (n=44), indigo bunting (n=34), red-eyed vireo (n=145), Swainson’s thrush (n=161) and wood thrush (n=50).
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