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Flight behaviour characteristics such as flight altitude and avoidance behaviour determine the species‐specific collision risk of birds with wind turbines. However, traditional observational methods exhibit limited positional accuracy. High‐resolution GPS telemetry represents a promising method to overcome this drawback. In this study, we used three‐dimensional GPS tracking data including high‐accuracy tracks recorded at 3‐s intervals to investigate the collision risk of breeding male Montagu's Harriers Circus pygargus in the Dutch–German border region. Avoidance of wind turbines was quantified by a novel approach comparing observed flights to a null model of random flight behaviour. On average, Montagu's Harriers spent as much as 8.2 h per day in flight. Most flights were at low altitude, with only 7.1% within the average rotor height range (RHR; 45–125 m). Montagu's Harriers showed significant avoidance behaviour, approaching turbines less often than expected, particularly when flying within the RHR (avoidance rate of 93.5%). For the present state, with wind farms situated on the fringes of the regional nesting range, collision risk models based on our new insights on flight behaviour indicated 0.6–2.0 yearly collisions of adult males (as compared with a population size of c. 40 pairs). However, the erection of a new wind farm inside the core breeding area could markedly increase mortality (up to 9.7 yearly collisions). If repowering of the wind farms was carried out using low‐reaching modern turbines (RHR 36–150 m), mortality would more than double, whereas it would stay approximately constant if higher turbines (RHR 86–200 m) were used. Our study demonstrates the great potential of high‐resolution GPS tracking for collision risk assessments. The resulting information on collision‐related flight behaviour allows for performing detailed scenario analyses on wind farm siting and turbine design, in contrast to current environmental assessment practices. With regard to Montagu's Harriers, we conclude that although the deployment of higher wind turbines represents an opportunity to reduce collision risk for this species, precluding wind energy developments in core breeding areas remains the most important mitigation measure.

Over-summering within or near the African wintering range by immature, non-breeding individuals occurs regularly in several species of long-distance migratory raptors, yet the extent of over-summering in Africa by adult birds remains unclear. In this study, we describe a case of an adult Montagu's Harrier over-summering in Africa, as revealed by GPS tracking. By relating detailed knowledge of the bird's movements to remotely sensed environmental data (normalized difference vegetation index, NDVI), we show that over-summering in this case was likely related to an exceptionally difficult breeding season the previous year rather than an effect of adverse weather conditions encountered during the winter or a failed attempt to migrate. Various factors are discussed as potential driving forces behind the bird's intra-African movements. Finally, we relate the documented case to a large number of North European Montagu's Harriers studied by telemetry and show that over-summering in Africa by adult individuals is indeed a rare event.

Time budgets are a powerful but hitherto seldom used way to study how migrants organise their bi‐annual travels. We studied daily time budgets of travelling Montagu's harriers Circus pygargus, based on GPS tracking data, in which we were particularly interested in how time budgets differ between regions and seasons, and are affected by wind. We found that Montagu's harriers used a relatively broad daily time window for travelling by starting daily travels just after sunrise and ending daily travels just before sunset. Occasionally, flights were extended into the night. Montagu's harriers frequently interrupted their daily flights for on average 1.5 h d–1. These interruptions occurred in all regions and seasons. The tracking data during interruptions suggested two different behaviours: in 41% of all interruptions the birds were moving (presumed foraging,) and in 32% they were stationary (presumed resting; the remaining interruptions could not be classified). The interruptions for foraging indicate that Montagu's harriers have a fly‐and‐forage migration strategy (i.e. combine travelling and foraging on the same day), but the interruptions for resting illustrate that their travels comprise of more than fly‐and‐forage behaviour alone. The large number of interruptions for foraging in the Sahara Desert indicates that this region is less hostile for a migrating raptor than presumed previously. Importantly, harriers spent more time on interruptions for resting on days with stronger headwinds, suggesting that interruptions for resting serve a function of waiting for more favourable weather conditions. Daily variation in time budgets was largely explained by wind; harriers flew more hours per day, and interrupted their flights fewer hours per day, on days they experienced stronger tailwinds. In contrast, time budgets were similar between regions and seasons, suggesting that wind rather than landscape and season shape travel routines of Montagu's harriers.

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.

Much of our knowledge about migratory behaviour comes from single-population tracking studies. Some such studies had a considerable impact on migration research at large. However, it is often unclear how representative such studies are for other populations, even of the same species. We compared migration corridors and schedules of GPS-tracked adult Western Marsh Harriers (Circus aeruginosus) from two nearby breeding areas within the West African–Eurasian flyway: the Low Countries (LC, N = 6) and southern Sweden (SW, N = 4). Assuming the migration patterns revealed by decade-old satellite-tracking of SW harriers are representative for the species, we expected LC harriers to make a narrow loop migration involving a western spring detour out of Africa, but according to an earlier schedule tuned to the earlier onset of spring in the Low Countries. In autumn, LC harriers migrated significantly further west than SW harriers all the way to their common non-breeding range. In spring, both groups detoured westward across the Sahara, as expected, and diverged towards their breeding areas after reaching mainland Europe. LC harriers migrated slightly earlier than SW harriers in autumn. However, LC harriers unexpectedly left their non-breeding sites up to a month before SW harriers, after which they made long stop-overs in northwest Africa. Late-departing SW harriers forewent these stop-overs, and thus caught up with LC harriers in northwest Africa, so that both groups reached their breeding areas simultaneously. While we anticipated strong overlap between LC and SW migration corridors in spring, we failed to anticipate the earlier and more variable spring departures of LC harriers. Early spring departures did not result in earlier arrivals by LC harriers. Instead, we suspect they departed early to escape faster deteriorating foraging conditions at their non-breeding sites. Such environmental modulation of migratory behaviour may complicate generalization of migration patterns in other birds.

Given that soaring birds travel faster with supportive winds or in good thermal soaring conditions, we expect weather conditions en route of migration to explain commonly observed regional and seasonal patterns in the performance of soaring migrants. We used GPS-loggers to track 13 honey buzzards and four Montagu's harriers for two to six migrations each. We determined how tailwinds, crosswinds, boundary layer height (a proxy for thermal convection) and precipitation affected hourly speeds, daily distances and daily mean speeds with linear regression models. Honey buzzards mostly travel by soaring while Montagu's harriers supplement soaring with flapping. Therefore, we expect that performance of harriers will be less affected by weather than for buzzards. Weather conditions explained between 30 and 50% of variation in migration performance of both species. Tailwind had the largest effect on hourly speeds, daily mean speeds and daily travel distances. Honey buzzards travelled significantly faster and farther, and Montagu's harriers non-significantly faster, under better convective conditions. Honey buzzards travelled at slower speeds and shorter distances in crosswinds, whereas harriers maintained high speeds in crosswinds. Weather conditions varied between regions and seasons, and this variation accounted for nearly all regional and seasonal variation in flight performance. Hourly performance was higher than predicted at times when we suspect birds had switched to intermittent or continuous flapping flight, for example during sea-crossings. The daily travel distance of Montagu's harriers was determined to a significant extent by their daily travel time, which differed between regions, possibly also due to weather conditions. We conclude with the implications of our work for studies on migration phenology and we suggest an important role for high-resolution telemetry in understanding migratory behavior across entire migratory journeys.

In many owl and raptor species, sexes have distinct parental roles. Females incubate the eggs and raise the chicks until independence, while males provide females and their chicks with food. This is believed to reduce sexual conflict over parental care as tasks do not overlap. The level of parental care is also shaped by parent-offspring conflict. The scarcity of empirical data on parental investment in species with sex-specific parental roles was our motivation to study parental care in the Montagu's Harrier Circus pygargus in relation to natural annual variation in food availability (vole abundance). By tracking individual birds using GPS-trackers, several aspects of parental care (the number of food provisioning trips, home range size and nest attendance) could be quantified for different nesting phases. We found that in food-poor years, males spent less time near the nest, and had lower food provisioning rates during the incubation and nestling phases. In addition, males had larger home ranges in food-poor years, a possible indicator of increased foraging effort. In contrast, females increased their contribution to food provisioning in food-poor years, as shown by higher food provisioning rates and larger home ranges. This increased foraging effort came at the cost of lower nest attendance by females. Our data suggest that, when food abundance declines, Montagu's Harriers shift from a system with almost strict sex-specific parental roles towards a system where both parents provide the same type of care with possibly increased sexual conflict.

Autumn migration routes of two Dutch female Montagu’s Harriers (Circus pygargus) were documented for the first time using satellite telemetry. Both migrated to their African wintering area—one via the Straits of Gibraltar through the Mediterranean and the other via Italy/Tunisia. The rate of travel was comparable to values reported for larger raptor species.

Loss and degradation of habitat threatens many bird populations. Recent rural land-use changes in the Netherlands have led to a shift in habitat use by breeding Montagu’s Harriers Circus pygargus . Since the 1990s, unprecedented numbers of this species have bred in farmland compared with numbers in natural habitat. Destruction of nests by agricultural operations, however, compromises breeding success. Between 1992 and 2005, the number of breeding pairs in the northeastern Netherlands was positively, though weakly, correlated with previous-year estimated abundance of voles, mostly Microtus arvalis . In good vole years, the onset of laying was earlier and mean clutch size was larger. Vole abundance was relatively higher in set-aside land and in high and dense vegetation. We suggest that agri-environmental schemes aimed at increasing the availability of voles in agricultural breeding areas may be an effective management tool for the conservation of Montagu’s Harriers in the northeastern Netherlands.

Farmland birds are in steep decline and agri-environment schemes (AES) to counteract these biodiversity losses are expensive and inefficient. Here we test a novel AES, Birdfields', designed using detailed ecological knowledge of the target species, Montagu's Harrier Circus pygargus. Current AES, such as field margins, that aim to improve foraging conditions (i.e. vole densities) for harriers are inefficient, as prey are difficult to capture in tall set-aside habitat. Birdfields' combines strips of set-aside to boost vole numbers and strips of alfalfa, as voles are accessible after alfalfa has been harvested. We found that vole numbers were generally highest in set-aside. Montagu's Harriers fitted with GPS-loggers used Birdfields' intensively after mowing, preferring mown to unmown strips. Thus, prey availability appeared more important than prey abundance. Thus, Birdfields', as a targeted AES for Montagu's Harriers, is more effective than previous AES due to increased prey accessibility. An additional advantage of Birdfields' is that it is considerably cheaper, due to the harvest of alfalfa. We advocate that AES should always include monitoring and research activities, aiming at a more adaptive conservation approach.

Knowledge about migratory connectivity, the degree to which individuals from the same breeding site migrate to the same wintering site, is essential to understand processes affecting populations of migrants throughout the annual cycle. Here, we study the migration system of a long-distance migratory bird, the Montagu's harrier Circus pygargus , by tracking individuals from different breeding populations throughout northern Europe. We identified three main migration routes towards wintering areas in sub-Saharan Africa. Wintering areas and migration routes of different breeding populations overlapped, a pattern best described by ‘weak (diffuse) connectivity’. Migratory performance, i.e. timing, duration, distance and speed of migration, was surprisingly similar for the three routes despite differences in habitat characteristics. This study provides, to our knowledge, a first comprehensive overview of the migration system of a Palaearctic-African long-distance migrant. We emphasize the importance of spatial scale (e.g. distances between breeding populations) in defining patterns of connectivity and suggest that knowledge about fundamental aspects determining distribution patterns, such as the among-individual variation in mean migration directions, is required to ultimately understand migratory connectivity. Furthermore, we stress that for conservation purposes it is pivotal to consider wintering areas as well as migration routes and in particular stopover sites.

Information about when and where animals die is important to understand population regulation. In migratory animals, mortality might occur not only during the stationary periods (e.g. breeding and wintering) but also during the migration seasons. However, the relative importance of population limiting factors during different periods of the year remains poorly understood, and previous studies mainly relied on indirect evidence. Here, we provide direct evidence about when and where migrants die by identifying cases of confirmed and probable deaths in three species of long-distance migratory raptors tracked by satellite telemetry. We show that mortality rate was about six times higher during migration seasons than during stationary periods. However, total mortality was surprisingly similar between periods, which can be explained by the fact that risky migration periods are shorter than safer stationary periods. Nevertheless, more than half of the annual mortality occurred during migration. We also found spatiotemporal patterns in mortality: spring mortality occurred mainly in Africa in association with the crossing of the Sahara desert, while most mortality during autumn took place in Europe. Our results strongly suggest that events during the migration seasons have an important impact on the population dynamics of long-distance migrants. We speculate that mortality during spring migration may account for short-term annual variation in survival and population sizes, while mortality during autumn migration may be more important for long-term population regulation (through density-dependent effects).

Summary 1. 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 (2005–2009). 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 017–027]. 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.