APalaearctic migratory raptor species tracks shifting prey availability within its wintering range in theSahel

Abstract

Summary1. Mid-winter movements of up to several hundreds of kilometres are typical for many migratory bird species wintering in Africa. Unpredictable temporary food concentrations are thought to result in random movements of such birds, whereas resightings and recoveries of marked birds suggest some degree of site fidelity. Only detailed (e.g. satellite) tracking of individual migrants can reveal the relative importance and the causes of site choice flexibility and fidelity. The present study investigates how mid-winter movements of a Palaearctic-African migratory raptor, Montagu's harrier Circus pygargus, in the Sahel of West Africa are related to the availability of food resources. 2. Thirty harriers breeding or hatched in northern Europe were satellite tracked (2005)(2006)(2007)(2008)(2009)). On average, four home ranges, each separated by c. 200 km, were visited during one overwinter stay in the Sahel. Wintering home ranges were similar in size to breeding season home ranges (average over wintering and breeding home range size c. 200 km 2 ), and harriers showed high site fidelity between years. 3. Most preferred habitat types in the Sahel were mosaics of grass-and cropland, indicating similar habitat preferences in both the breeding-and wintering seasons. 4. The main prey of Montagu's harriers in the Sahel were grasshoppers Acrididae. Highest grasshopper numbers in the field occurred at relatively low vegetation greenness [normalized difference vegetation index (NDVI) values 0Á17-0Á27]. We used NDVI as a proxy of food availability for harriers. 5. During their overwinter stay, Montagu's harriers moved in a South-South-western direction between consecutive home ranges. The birds selected areas within the range of NDVI values associated with high grasshopper numbers, thus tracking a 'green belt' of predictable changes in highest grasshopper availability. 6. Contrary to earlier hypotheses of random movements in the Sahelian-wintering quarters, the present study shows that Montagu's harriers visited distinct home ranges, they were sitefaithful and tracked seasonal changes in food availability related to previous rainfall patterns, caused by the shifting Intertropical Convergence Zone. Itinerancy may be the rule rather than an exception among insectivorous birds wintering in African savannahs.

Migration of the Common Redstart (Phoenicurus phoenicurus)

“…Trierweiler et al (2013) reported that individual Montagu's Harriers (Circus pygargus) followed patterns of vegetation presence that presumably reflect food availability, and a similar pattern has been found in Lesser Kestrels (Falco naumanni; Catry et al 2011). Less is known of itinerancy in passerines.…”

Resource tracking within and across continents in long-distance bird migrants

“…However, the hypothesized link between movements and seasonal regional resources throughout the annual cycle has never been properly established, and the precise timing of annual individual schedules relative to seasonally induced local dynamics in habitats, food resources, and climate remains restricted to parts of the annual cycle in long-distance migrants ( 6 – 8 , 13 , 15 , 16 , 25 ). In order to establish potential links between resources and annual bird movements, we have combined the results of high–temporal resolution tracking of individual birds with monitoring variation in local ecological conditions across the seasonally changing globe.…”

“…The accelerating vegetation growth during spring green-up provides another potentially high food availability, also known as surfing the green wave ( 6 , 13 ), although, for insectivores, the food peak is generally delayed. Avoiding decreasing vegetation conditions could also drive movements ( 15 , 16 ). …”

Terrestrial animal tracking as an eye on life and planet

“…A particularly exciting prospect is the increasingly detailed and complete environmental annotation of movement paths. MODIS sensors have the potential to provide global information on greenness and other land-surface attributes down to 8-day or even daily frequency, and researchers have begun to successfully use these to relate movement to resource availability (73). Remote-sensing-supported predictions of climate conditions in daily and 1-km resolution (74) hold the promise to extend such annotations to ecologically even more meaningful variables.…”