Despite the wealth of studies on seasonal movements of birds between southern nonbreeding locations and High Arctic breeding locations, the key mechanisms of navigation during these migrations remain elusive. A flight along the shortest possible route between pairs of points on a sphere (‘orthodrome’) requires a bird to be able to assess its current location in relation to its migration goal and to make continuous adjustment of heading to reach that goal. Alternatively, birds may navigate along a vector with a fixed orientation (‘loxodrome’) based on magnetic and/or celestial compass mechanisms. Compass navigation is considered especially challenging for summer migrations in Polar regions, as continuous daylight and complexity in the geomagnetic field may complicate the use of both celestial and magnetic compasses here. We examine the possible use of orientation mechanisms during migratory flights across the Greenland Icecap. Using a novel 2 g solar-powered satellite transmitter, we documented the flight paths travelled by a female red knot (Calidris canutus islandica) during two northward and two southward migrations. The geometry of the paths suggests that red knots can migrate across the Greenland Icecap along the shortest-, orthodrome-like, path instead of the previously suggested loxodrome path. This particular bird’s ability to return to locations visited in a previous year, together with its sudden course changes (which would be appropriate responses to ambient wind fields), suggest a map sense that enables red knots to determine location, so that they can tailor their route depending on local conditions.

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Article: A red knot as a black swan: how a single bird shows navigational abilities during repeat crossings of the Greenland Icecap

Journal: Journal of Avian Biology

Article DOI: 10.1111/jav.02464

Authors: Eva M. A. Kok*, T. Lee Tibbitts, David C. Douglas, Paul W. Howey, Anne Dekinga, Benjamin Gnep & Theunis Piersma. *Corresponding author: E-mail: evamakok@gmail.nl, ORCID-ID: 0000-0002-3961-008X, ORCID-ID: Theunis Piersma: 0000-0001-9668-466X.

1. Raw tracking data: rekn13_totloc_r130d10lc1.txt

All Argos data were filtered by the SAS version of the Douglas Argos Filter algorithm using the following parameter values: maxredun = 10 km, maxrate = 130 kph, and coef = 25. Locations that met the distance-angle-rate filter thresholds are labeled '1' in the  weeddist column.

2. Identification of departure and arrival locations: lables JAB.csv

See Material and Methods/Spatial analyses: Departure and arrival times and the intervening flights were determined from the visual inspection of the tracks (Google Earth ©). Departure locations were defined as the last location on land with a ground speed < 20 km/h (Chan et al. 2019) and arrival locations as the first location on land with ground speed < 20 km/h. Intermediate locations were defined as ‘migratory flights’ and were labeled as ‘northward’ or ‘southward’ migration together with a unique identifier for each migratory flight by year (number of migratory flights per year is 4 for 2016, and 5 for 2017, see Figure 1 and Supplementary material Appendix 1, Table A1 for the migration scheme for both years).

3. R script to extrapolate orthodromes and geographic and magnetic loxodromes: Koketal2020 JAB script.R.

See Material and Methods/Orthodrome or loxodrome routes? : Geographic orthodrome and loxodrome paths between Iceland and Ellesmere Island were simulated by means of the functions gcIntermediate() and destPointRhumb() of the geosphere package (Hijmans 2019), with a resolution of 1 point per kilometer between previously assigned departure and arrival locations. Since Paula’s tracks crossed Iceland without stops during both southward migrations (a pattern predicted by Dietz et al. 2010), we assigned ‘pseudo-‘ arrival locations and arrival times by means of the locations where she first crossed the Icelandic coastline and the average groundspeeds of the respective migratory flights.