DOI: 10.5061/dryad.9cnp5hqg5
title: Earlier and slower or later and faster: Spring migration pace linked to departure time in a Neotropical migrant songbird

description: 1. Migratory birds travel vast distances and the timing of migratory
 flights can affect survival and the ability to reproduce. For Neotropical
 migrant songbirds, early spring departure from wintering sites, early
 arrival to the breeding grounds and higher reproductive success have been
 related to the use of suitable habitats and environmental conditions
 during the non-breeding season. However, how migratory strategies are
 shaped by winter habitat choice is largely unknown due to the general
 inability to track birds from specific wintering habitats to stopovers or
 breeding destinations. 2. We assessed how winter habitat (native forest
 vs. shade-grown coffee plantations) relates to spring departure date and
 migration pace in Swainson’s Thrush (Catharus ustulatus). We also
 determined the effect of departure date and total migration duration on
 the arrival date of birds detected near or within their breeding range. 3.
 We used a novel application of Motus radio telemetry arrays to track
 individuals from their wintering grounds in the Andes of South America
 along their migratory journey to North America. 4. We found variation in
 migratory strategies between habitats, with birds wintering in native
 forest departing later than birds in coffee. We present isotopic evidence
 for native forest being of higher quality than shade coffee for Swainson’s
 Thrush and hypothesize that moister conditions in forest, as shown by
 stable isotope (δ13C) analysis of thrush whole blood, provides favourable
 pre-migratory conditions allowing birds to delay departure from wintering
 grounds. Habitat, between-site and -year variation in departure date,
 suggests that birds made facultative adjustments to winter habitat quality
 and environmental conditions. Independent of habitat, birds that departed
 later migrated faster and this pattern was maintained along the migration
 route (n = 44). Migrating earlier and slower or later and faster was
 unlikely to result in significant differences in arrival time to breeding
 destinations.  5. Our findings reveal underappreciated
 complexity in migratory decisions by long-distance migrants that contrast
 with the current paradigm of earlier departures and arrival from optimal
 habitats. The next step is to understand the relative fitness benefits of
 early versus late schedules or whether each strategy is an equally good
 response to experienced conditions.
description: We captured
 Swainson’s Thrush (<i>Catharus ustulatus</i>) between 10
 January and 30 March from 2014 to 2018 at two study sites located on the
 western slope of Colombia’s Eastern Andes (Hacienda La Fragua and Los
 Vientos). We captured birds in shade-coffee plantations and secondary
 forests at each site. Birds were banded, processed, and fitted with
 digitally-coded VHF transmitters using leg-loop harnesses made of elastic
 thread. We tracked birds locally and during spring migration using an
 array of over 300 automated radio-telemetry stations spanning key
 migratory flyways. We used Motus to determine the effect of occupying
 native forest or shade-grown coffee during the overwintering season on
 spring migration departure date (Data set 1) and migration pace (Data set
 2) of a Nearctic-Neotropical migratory landbird, the Swainson’s Thrush
 (<i>Catharus ustulatus</i>). We also determined the effect of
 departure date and total migration duration on the arrival date of birds
 detected near or in their breeding range (Data set 3). To describe the
 relative quality of shade coffee versus native forest, we
 measured<i> </i>stable carbon (δ<sup>13</sup>C)
 and nitrogen (δ<sup>15</sup>N) isotopes in whole blood to
 assess moisture patterns and the relative contribution of fruit and
 insects as sources of dietary protein (Data set 4).