DOI: 10.5061/dryad.r7sqv9snq
title: Age-related differences in fall migration timing and performance of juvenile and adult Wood Thrushes departing from a breeding site

description: Juvenile passerines are expected to have lower migration performance than
 adults due to their inexperience with long-distance flights and
 morphological limitations, such as shorter wing length. From 2016-2019 we
 radio-tagged nestling and adult Wood Thrushes (Hylocichla mustelina) at a
 breeding site in southwestern Ontario and used the automated Motus
 Wildlife Tracking System to test if age class predicts timing of the onset
 of fall migration (date, time of night), flight speed during the initial
 migration flight across Lake Erie, and overall pace of migration southward
 through the eastern United States. We detected 60/117 (51%) adults and
 82/119 (69%) juveniles departing the breeding area as they initiated fall
 migration. Compared with adults, juveniles departed significantly earlier
 in fall and significantly later in the evening. When crossing Lake Erie on
 their first migration flight juveniles travelled about 25% slower than
 adults but this was due primarily to adults making better use of
 tailwinds. When travelling south through the eastern U.S. juveniles had a
 significantly slower overall migration pace (47.3 ± 5.1km/day) than adults
 (71.6 ± 4.7km/day). Although we found some evidence that juvenile Wood
 Thrushes have lower fall migration performance than adults, identifying
 the proximate and ultimate mechanisms remains a challenge. There is no
 evidence that juvenile Wood Thrushes are inefficient in migration flight
 or refueling at stopovers, and it is unlikely that the fall migration pace
 in this species affects their ability to compete for wintering food
 resources. More tracking studies from breeding sites are needed to
 understand the ecological factors favouring and biological significance
 of, age-related differences in migration performance.
description: <em>Radio-tagging</em> When
 nestlings were 10 days old, shortly before fledging which occurs at 12-14
 days, they were removed from the nest and banded with both Canadian
 Wildlife Service numbered aluminum bands and unique color band
 combinations. The largest nestling (by mass) had a blood sample
 (&lt;25 uL) taken for genetic sexing (HealthGene Molecular Diagnostic
 and Research Center, Concord, Canada) and was equipped with a uniquely
 coded radio transmitter using a backpack leg loop harness (Rappole and
 Tipton 1991) made from 2.5mm Teflon tubing. Only one nestling was tagged
 at most nests (82%; 131 of 160 nests) but two nestlings were tagged at 29
 nests that fledged late in the season. Of all tagged nestlings (n = 189),
 133 survived to 16 days post-fledging and 119 survived to the onset of
 fall migration. We refer to this age class as juvenile.
 Adults were caught by placing mist nets in the vicinity of nests
 and then banded with both Canadian Wildlife Service numbered aluminum
 bands and unique color band combinations. As with the nestlings, breeding
 adults (n = 117; 32 male and 85 female) were equipped with a uniquely
 coded radio transmitter using a backpack leg loop harness (Rappole and
 Tipton 1991) made from 2.5mm Teflon tubing. Sex was determined by checking
 for the presence of a brood patch or cloacal protuberance and age was
 determined as either second year or after second year using plumage
 criteria (Pyle 1997). The specifications of the
 radio-tags varied among years (2016: NTQBW-6-1 (1.6g); 2017: ANTC-M6-1
 (1.7g);  2018: ANTC-M6-1 and NTQB2-6-1 (1.6g); 2019: NTQB2-4-2S (1.5g))
 due to the manufacturer (Lotek Wireless Inc., Newmarket, Canada)
 discontinuing models, but all had similar performance, a burst rate of
 12.7 seconds, and minimum expected lifespan of 400 days.
 <em>Motus Migration Detections</em>
 Motus receivers sometimes record false detections due to random
 radio noise, duplicate tags, and overlapping tag signals when multiple
 tags are transmitting in the same area (Crewe et al. 2018). A number of
 filtering and quality control steps were taken to identify these and
 exclude them from analysis. First, we eliminated detections of fewer than
 3 consecutive tag bursts because they are likely to be false detections
 (Crewe et al. 2018). Next, we eliminated any detection that occurred in
 impossible locations based on prior knowledge of migration timing and
 routes from geolocator studies (Stutchbury et al. 2011, Stanley et al.
 2015). <strong> </strong>Detections
 representing the initiation of fall migration were those that occurred
 after sunset during the migratory period (August 25 to October 15), when a
 bird was not detected in Ontario again until the following year. We used
 the time stamp of the first detection on the migration date to represent
 the time of day of the start of migration. Because the timing of sunset
 (the moment the sun disappears below the horizon) varies greatly during
 the migratory period, we calculated the number of minutes after sunset
 each bird began its migration at and used this number as our response
 variable. Motus towers cannot determine when a bird actually begins a
 migratory flight, only when it is first detected. For this reason, we
 removed one extreme outlier from analysis, an adult that was detected 84
 minutes later than the next latest bird, more than 5 hours after
 sunset. Our study area lies on the north shore of Lake
 Erie which gave us the unique opportunity to estimate flight speed of the
 initial migratory flight of birds that were detected departing the study
 area and subsequently detected along the south shore of Lake Erie. We
 cannot assume that the movements between stations are linear, but we can
 be reasonably certain that birds did not land in the water between
 detection at these towers. Signal strength readings when a bird flies past
 a tower create a parabolic arc with increasing strength heading toward the
 tower and decreasing strength after passing the tower. Because there is
 variation in the range at which a tower can detect a bird, we calculated
 the amount of time it took a bird to fly from peak signal strength at the
 last tower on the north shore of Lake Erie to the peak signal strength at
 the first tower on the south shore of Lake Erie (Begin-Marchand et al.
 2021; Additional Figure 1.). We measured the distance between the two
 towers and using the time elapsed between peak detections, calculated the
 average flight speed (km/h) and used this as our response variable. Some
 birds were detected before and after crossing Lake Erie, but were not
 picked up by one or both towers long enough to show the expected rising
 and falling of signal strength. These individuals (n=11 adults; 8
 juveniles) were removed from analysis because without a clear peak in
 signal strength, we could not accurately estimate the time when they
 passed each tower. The direction and speed of wind was
 found to be an important predictor of migratory flight speed (Mitchell et
 al. 2015) so we included it as a covariate in our flight speed models.
 Following the methods of Morbey et al. 2018, we used the RNCEP package
 (Kemp et al. 2012) in R to extract easterly (90°) and northerly (0°) wind
 components at the 925mb pressure level, which corresponds to an altitude
 of 675­–825m over Lake Erie and is comparable to the altitude of migratory
 flights measured in other species of thrushes (Bowlin et al. 2015). We
 used linear interpolation with the <em>NCEP.interpol</em>
 function to interpolate 925mb wind conditions at the time and date of
 departure for each bird. To estimate the tailwind component, we used
 <em>Vw </em>× cos(β), where <em>Vw </em>is the
 wind speed and β is the difference between the flight bearing and wind
 direction (Safi et al. 2013). We assumed a flight bearing of 180°, as most
 birds were detected almost directly south of the study area after crossing
 the lake. This resulted in a tailwind metric that ranged from -10 when
 flying directly into a strong headwind, to 10 when benefitting from a
 strong and direct tailwind. The last measure we
 examined was the pace of migration through the U.S. after the night of the
 initial migratory flight. Motus does not allow us to determine when a bird
 is stopping over (e.g., relatively stationary for one or more nights)
 versus migrating because of gaps in tower coverage, but we can measure the
 time it takes a bird to travel between towers and measure the distance
 between those towers to calculate the average number of kilometers a bird
 travels per day. If a bird is spending more time stopping over, it will
 travel fewer kilometers per day when covering the distance between towers.
 To ensure that we were measuring stopover behavior and not speed of direct
 overnight flights, we eliminated detections that occurred on consecutive
 days (n=2). We detected 25 individuals at multiple towers, not on
 successive days, as they migrated south through the United States. In
 Swainson’s Thrushes (<em>Catharus ustulatus</em>) migration
 pace was found to be slower at higher latitudes (Begin-Marchand et al.
 2021), so we calculated the midpoint in latitude between Motus towers to
 include as a variable in analysis. We also included the distance between
 Motus towers because it was also found to be important to predicting
 migration pace in Swainson’s Thrushes, with a higher pace when detections
 were geographically closer to each other (Begin-Marchand et al.
 2021).
description: # Age-related differences in fall migration timing and performance of
 juvenile and adult Wood Thrushes departing from a breeding site
 [https://doi.org/10.5061/dryad.r7sqv9snq](https://doi.org/10.5061/dryad.r7sqv9snq) ## Description of the data and file structure This data was collected during the summers of 2016-2019 in Norfolk County, Ontario, Canada. Wood Thrushes were captured in a variety of forest fragments and tagged using coded Nanotags designed to work with the automated telemetry Motus Wildlife Tracking System. To attach tags to birds, adult Wood Thrushes were captured in mist nets and juvenile Wood Thrushes were briefly removed from the nest at 10 days old, shortly before fledging. We used the time and location of detections recorded by the Motus system to populate the spreadsheets in this dataset. The data have been uploaded in three separate spreadsheets and the title of each spreadsheet identifies the analysis that it was used for. ### Files and variables #### File: Boyd\_et\_al.\_migration\_departure\_date\_and\_time.xlsx **Description:** This dataset was used to complete analysis of factors influencing date and time of migration departures. ##### Variables * tag ID - unique ID assigned to each radio tag * sex - sex of bird * age - age of bird * year - year of detection * tower - name of tower that detected bird * latitude - latitude of tower that detected bird * longitude - longitude of tower that detected bird * detection date - date bird was detected * detection time (EST) - time of day bird was detected * time of sunset - time of sunset on day bird was detected * difference between sunset and detection - hours and minutes between sunset and detection * minutes between sunset and detection - hours and minutes converted to minutes between sunset and detection #### File: Boyd\_et\_al.\_flight\_speed.xlsx **Description:** This dataset was used to complete analysis of factors influencing flight speed during the first migratory flight. ##### Variables * tag ID - unique ID assigned to each radio tag * sex - sex of bird * age - age of bird * year - year of detection * south shore tower - name of tower that detected bird on south shore of Lake Erie * south tower lat - latitude of south shore tower * south tower long - longitude of south shore tower * north shore tower - name of tower that detected bird on north shore of Lake Erie * north tower lat - latitude of north shore tower * north tower long - longitude of north shore tower * detection date - date bird was detected  * detection time (EST) - time bird was detected * north max signal time - time of highest signal strength at north tower * south max signal time - time of highest signal strength at south tower * flight time between towers - time between peak signal strength at north and south towers in minutes and hours * flight time in minutes - time between peak signal strength at north and south towers in minutes * distance between towers (km) - linear distance between north and south towers * average flight speed (kmh) - average flight speed based on distance and time between towers * tailwind metric - metric incorporating wind speed and direction to estimate advantage gained by wind #### File: Boyd\_et\_al.\_migration\_pace.xlsx **Description:** This dataset was used to complete analysis of factors influencing pace of migration across the continental USA. ##### Variables * tag ID - unique ID assigned to each radio tag * sex - sex of bird * age - age of bird * year - year of detection * start tower - name of northern tower where bird was detected * end tower - name of southern tower where bird was detected * starting latitude - latitude of start tower * mid latitude - latitude halfway between start and end towers * ending latitude - latitude of end tower * detection date - date bird was detected at end tower * time between detections (hours) - time between detections at start and end towers in hours * time between detections (days) - time between detections at start and end towers in fractions of days * distance between detections (km) - linear distance between start and end towers * distance covered per day (km) - average distance covered per day based on timing of detections and distance between start and end towers ## Code/software The data is uploaded in .xlsx files and can be viewed in Microsoft Excel or Program R. ## Access information Other publicly accessible locations of the data: * Raw detection data from the Motus Wildlife Tracking System available at [https://motus.org/](https://motus.org/) under project 46.