DOI: 10.5061/dryad.144sv
title: Data from: How individual Montagu's Harriers cope with Moreau's Paradox during the Sahelian winter

description: Hundreds of millions of Afro-Palaearctic migrants winter in the Sahel, a
 semi-arid belt south of the Sahara desert, where they experience
 deteriorating ecological conditions during their overwintering stay and
 have to prepare for spring migration when conditions are worst. This
 well-known phenomenon was first described by R.E. Moreau and is known ever
 since as Moreau's Paradox. However, empirical evidence of the
 deteriorating seasonal ecological conditions is limited and little is
 known on how birds respond. Montagu's Harriers Circus pygargus spend
 6 months of the year in their wintering areas in the Sahel. Within the
 wintering season, birds move gradually to the south, visiting several
 distinct sites to which they are site-faithful in consecutive years. At
 the last wintering site, birds find themselves at the southern edge of the
 Sahelian zone and have no other options than facing deteriorating
 conditions. We tracked 36 Montagu's Harriers with GPS trackers to
 study their habitat use and behaviour during winter and collected data on
 the abundance of their main prey, grasshoppers, in Senegal. Since
 grasshopper abundance was positively related to vegetation greenness
 (measured as normalized difference vegetation index, NDVI), we used NDVI
 values as a proxy for prey abundance in areas where no field data were
 collected. Prey abundance (grasshopper counts and vegetation greenness) at
 wintering sites of Montagu's Harriers decreased during the wintering
 period. Montagu's Harriers responded to decreasing food availability
 by increasing their flight time during the second half of the winter.
 Individuals increased flight time more in areas with stronger declines in
 NDVI values, suggesting that lower food abundance required more intense
 foraging to achieve energy requirements. The apparent consequence was that
 Montagu's Harriers departed later in spring when their final
 wintering site had lower NDVI values and presumably lower food abundance
 and consequently arrived later at their breeding site. Our results
 confirmed the suggestions Moreau made 40 years ago: the late wintering
 period might be a bottleneck during the annual cycle with possible
 carry-over effects to the breeding season. Ongoing climate change with
 less rainfall in the Sahel region paired with increased human pressure on
 natural and agricultural habitats resulting in degradation and
 desertification is likely to make this period more demanding, which may
 negatively impact populations of migratory birds using the Sahel.
description: Schlaich_etal_2016_JAE_gps_tracking_data_20160802This file contains the GPS-tracking data of Montagu’s Harriers used for analyses on behavior during the wintering season and contains the following columns: season (name of the bird, phase of annual cycle, years), date_time (date and time in UTC), latitude, longitude, speed_2d (instantaneous speed measured by the GPS-device).Schlaich_etal_2016_JAE_grasshopper_count_data_20160802This file contains the grasshopper count data collected in 2014 and 2015 at five sites in Senegal and contains the following columns: year, month, area, id (grid point ID, transect ID), lat.start, long.start, lat.end, long.end (coordinates of starting and end points of transects), date_time, grasshoppers_small, grasshoppers_big (number of grasshoppers counted 1 m left and right of transect line).