How individual Montagu's Harriers cope with Moreau's Paradox during the Sahelian winter

Abstract

Summary 1.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. 2. 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. 3. 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. 4. 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. 5. 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.

Disentangling the effects of environmental conditions on wintering and breeding grounds on age‐specific survival rates in a trans‐Saharan migratory raptor

“…Harriers breeding in western Europe spend the wintering season (October-March) in the western Sahel (Limiñana et al 2012, Trierweiler et al 2013, where they prey mostly on grasshoppers (Trierweiler and Koks 2009, Trierweiler et al 2014, Augiron et al 2015, Schlaich et al 2016. The Montagu's harrier is a long-distance migrant breeding across the open farming landscapes of the western Palearctic, from northern Morocco to western Russia and Kazakhstan.…”

Methods in field chronobiology

“…bees, fish in small ponds, birds), lifelong tracking of individuals (e.g. using RFID and satellite tracking; see Case study 1 [27]) is already possible. Information on individual variation in circadian organization, in combination with data on longevity will provide new insights on the evolutionary consequences of daily rhythms in free-ranging animals (table 1, question 5).…”

“…Thus, in this particular example, activity patterns tended to be similar across individuals at a given site, but varied to a greater degree across sites (figure 2), possibly suggesting a prominent role of environmental drivers shaping activity patterns in wild Montagu's Harriers. Seasonal differences in daily activity patterns could, for instance, arise from different feeding habits of Montagu's Harriers in the three different seasons: voles and the need to feed young during the breeding season [57], eggs and nestlings of passerines during the main spring stopover in northwest Africa [27], and grasshoppers in winter [44]. Whether between-individual variation in activity patterns reflects differences in individual personalities, with some birds being more explorative than others (e.g.…”

Advances and Environmental Conditions of Spring Migration Phenology of American White Pelicans

“…Song birds and insectivorous raptors in the habitat of higher food availability and more rainfall depart earlier than those in the habitat of lower food availability and less rainfall3637. The NDVI has been used to represent the ecological and habitat conditions of wintering and stopover habitats to predict the migration dates of terrestrial insectivorous birds141838.…”

“…that the importance of the NDVI during the entire wintering stay was confounded by the strong effect of year in their study. Schlaich et al 37. found that Montagu’s harriers ( Circus pygargus ) increased flying and foraging time in the habitat of low NDVI and grasshopper (prey) abundance and subsequently delayed spring departures from the wintering grounds in the Sahel.…”