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Rapid recovery by fat- and muscle-depleted Blackpoll Warblers following trans-oceanic migration is driven by time-minimization


Bayly, Nicholas J. et al. (2021), Rapid recovery by fat- and muscle-depleted Blackpoll Warblers following trans-oceanic migration is driven by time-minimization, Dryad, Dataset,


Non-stop endurance flights are a defining characteristic of many long-distance migratory birds but subsequent recovery phases are not typically distinguished from fueling phases (collectively ‘stopovers’), despite endurance flights inducing marked physiological changes including flight muscle atrophy and gastrointestinal tract reductions. Here, we hypothesize that recovery requires unique behavioral adaptations, leading to departures from the predictions of optimal migration theory for time-minimizing migrants. We predict that recovering birds will 1) select (moist) food-rich habitats on arrival; 2) have slow initial fueling rates due to decreased gastrointestinal capacity; 3) show a negative correlation between stopover duration and arrival condition instead of a negative correlation with fuel deposition rate (FDR); 4) stopover longer than required to store energy reserves for subsequent flights; and 5) show evidence of rebuilding flight muscles. To test these predictions, we studied Blackpoll Warblers Setophaga striata in northern Colombia following trans-oceanic flights >2250 km. Birds selected dry seasonal habitats, despite proximity of moist forests, and among 1227 captured individuals, 14-21% were emaciated and 88% had atrophied flight muscles. We recaptured 74 individuals, revealing net positive mass gains and, contrary to Prediction 2, no evidence for slow initial recovery rates. Contrary to Prediction 3, stopover duration was only weakly correlated with arrival condition and birds with high FDR (4.9% LBM/day) had shorter durations (3 days) relative to birds with slower rates (7 days) - both groups accumulated sufficient fuel to reach non-breeding (over-wintering) grounds 500-1000 km away. Mass increases were largely attributable to fat deposition but some birds improved flight muscle condition (31.9%), consistent with Prediction 5. Together these results reveal a strong selection for time-minimization in the decisions made by Blackpoll Warblers following trans-oceanic flights, likely mediated through advantages to early arrival on non-breeding grounds, contrary to our hypothesis of recovery imposing unique selection pressures.


This dataset consists of data collected using two methods:

1. Occupancy surveys carried out between September-November in 2016 (northern Colombia) and 2017 (Panama) across 21 study sites with associated vegetation data. For the data presented here, 5748 repetitions of 264 individual transects were used, where each transect was 100 m long and surveyed during 10 minutes. An average of four repeat visits were made to each transect every 10 days during six 10-day periods between September and November. These data have been processed to create an observation history for Blackpoll Warblers in each transect, along with associated vegetation and climatic data for each.

2. A constant effort mist-netting station was established in the El Pantano reserve on the Guajira Peninsula of Colombia during two fall migrations. Mist-nets were opened daily between 6 am and 11 am and captured birds were banded and data were taken on species, age, sex, physical condition, wing and weight. Data included here only include captures of Blackpoll Warblers the focal species of the study. Subsequent processing was carried out to model body mass change in recaptured birds and stopover durations (see methods in original article).

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