Module 5: Migration & Navigation

Of the ~350 species of diurnal raptors, about 60 are long-distance migrants. The flagship Palearctic-African migrants include:

• Aquila nipalensis (steppe eagle) — breeding from Russia to Mongolia, wintering in the African Rift Valley and the Arabian Peninsula; ~6000 km one-way.
• Clanga pomarina (lesser spotted eagle) — Eastern-European breeder wintering in Southern Africa; the longest-distance migrant of the genus.
• Clanga clanga (greater spotted eagle) — slightly larger, less obligately migratory; individuals cross the Bosphorus and Levantine flyway.
• Pernis apivorus (European honey buzzard) — famous for its thermal-soaring dependency and bottleneck concentrations at Gibraltar.
• Circus harriers, Circaetus gallicus (short-toed snake eagle), and the New-World Buteo swainsoni (Swainson’s hawk).

Movement is strongly seasonal and tied to thermal season. Adult steppe eagles often travel 200–400 km per day during peak migration by alternating climbs and glides, accumulating altitude in each thermal and then sacrificing it over many kilometres of inter-thermal glide.

Bottleneck sites

Thermals are driven by solar heating of dry land and collapse over water. Migrating raptors therefore concentrate at narrow land bridges:

• Bosphorus (Turkey): 15–20 km strait; hundreds of thousands of broad-winged raptors pass in autumn.
• Bab-el-Mandeb (Red Sea entrance):30 km; the primary land-connection between East Africa and Eurasia for Steppe-eagle-class migrants.
• Gibraltar: 14 km; Western European autumn funnel; Clanga rare but Pernis, Buteo, and Circaetus abundant.
• Veracruz (“river of raptors”):Gulf-coast Mexico, bottleneck for Buteo, Ictinia, and Swainson’s hawk; annual counts exceed 5 million.

The glider analogue of raptor soaring was formalised by MacCready (1958) for competition sailplane pilots and adapted to birds by Pennycuick (1972, 1975, 1989). An eagle alternates between two phases: (A) climbing spirally in a thermal updraught of vertical velocity \( w_t \), and (B) gliding inter-thermal along a glide polar that trades speed for sink rate.

Glide polar

At airspeed \( V \), the bird’s vertical sink rate is:

MacCready speed-to-fly

For a cycle of climb rate \( w_t \) followed by glide at \( V \) with sink \( w_s(V) \), the average ground speed is:

Minimum-power vs. maximum-range speed

The mechanical flight power (Pennycuick 1989) is the sum of parasitic, induced, and profile contributions:

Slope-glide uplift and sky-sampling

In hilly terrain, ridges generate deflection uplift that eagles exploit as an auxiliary lift source (sky-sampling during slope-soar; Bohrer et al. 2012). The “river of raptors” at Veracruz is driven by these ridge-lift sources through the coastal plain. Strouhal frequency of flap-soar alternation in long-distance migrants (St = fA/U) falls in the 0.2–0.4 range characteristic of efficient flapping flight (Taylor et al. 2003).

A migrating raptor leaves its natal territory and accurately finds a wintering area it has never seen. The answer to “how?” is a hierarchical navigation system drawing on at least three compasses — magnetic, solar, and landmark — integrated onto an underlying cognitive map.

Magnetic compass: radical-pair mechanism

Wiltschko & Wiltschko (1972) first demonstrated that migrating European robins respond to the inclination of the magnetic field rather than its polarity. Three decades of work identified the retinal flavoprotein cryptochrome (Cry4 in birds) as the light-activated sensor. Upon blue-light excitation, a flavin– tryptophan electron transfer creates a short-lived radical pair whose singlet/ triplet recombination probability depends on the external magnetic field:

Sun compass with time compensation

Kramer (1952) demonstrated that birds use the sun’s azimuth, corrected by the internal circadian clock. The sun-compass correction is:

Landmark recalibration at night-roost

Night-roost sites along the flyway — traditional forests at the edge of mountain ranges, or prominent coastal promontories — provide visual reference points that recalibrate the magnetic heading estimate. Grüter & Leadbeater (2014) model this as a Bayesian update:

GPS telemetry: from VHF to Argos

Viitala et al. (1995) pioneered the use of small VHF transmitters on birds of prey to reveal UV markings on rodent urine trails (a vole-hunting cue). Modern studies use Argos/GPS transmitters (20–80 g) mounted as backpack harnesses, producing sub-kilometre positional fixes at 10–30 minute intervals. The result is an unprecedented picture of individual-level migration: Meyburg et al. (2017) documented the flyway of a single lesser spotted eagle across 9 calendar years and 70 000 km of cumulative flight.

Soaring is energetically cheap, typically 2–4 times the basal metabolic rate. Flapping flight over water is 20–30 times basal. Therefore the ecological currency that drives bottleneck routing is energy, not distance: a 2000 km land route that can be soared is cheaper than a 500 km direct water crossing that forces powered flight.

Energy budget over a crossing

For a steppe eagle making a direct crossing of width \( d \) with wind speed \( u(t) \) (positive tailwind):

Broad-front vs. narrow-front

When thermal availability is strong and fat reserves are high, some individuals choose a broad-front water crossing rather than the narrow-front land bypass. The equilibrium proportion of broad-front migrants is an evolutionarily stable strategy determined by the distribution of wind regimes and the cost of the detour.

Both et al. (2006) documented that spring arrival dates for several European long-distance migrants have advanced by 10–15 days over the past four decades, matching the earlier green-up of temperate spring. Mismatch between predator arrival and prey availability is a well-documented driver of population decline in some species (e.g., Pernis apivorus in the Netherlands). Stopover habitat loss — particularly drying of the Sahelian stopover zone south of the Sahara — represents a second, independent pressure.

Vulture migration

Old World vultures (Gyps, Aegypius) are among the highest- soaring long-distance movers on the planet. Gyps rueppellii (Rüppell’s griffon) holds the bird altitude record at 11,274 m, a collision altitude inferred from the ingestion of one such vulture by a commercial aircraft over Côte d’Ivoire (Laybourne 1974). Their cross-continent movements follow the same MacCready glide-and-thermal template as accipitrid eagles and share many bottleneck sites (Bildstein 2006).

Long-distance migrants must switch between pure gliding, thermal-soaring, and powered flapping as conditions dictate. The transition is not continuous: at a given wing loading, flapping below some minimum speed becomes energetically wasteful, and at speeds above the maximum-range speed flapping is again inefficient. The resulting gait-transition diagram has been formalised by Rayner (1979) and refined for raptors by Tobalske (2007).

Strouhal number of flap-soar

During flap-and-glide, the wingbeat has characteristic amplitude \( A \) and frequency \( f \); the non-dimensional Strouhal number is:

Wing loading and aspect ratio

Wing loading \( W/S \) (weight over wing area) and aspect ratio \( \text{AR} = b^2/S \) determine the shape of the glide polar. Aquila nipalensis has a relatively high wing loading (~80 N/m²) and a moderate aspect ratio (~6.5) — trade-offs that favour fast gliding and strong-thermal soaring over dead-low-speed manoeuvring. By contrast Clanga pomarina has lower wing loading (~55 N/m²) and similar AR, allowing it to climb in weaker thermals — a decisive advantage on the autumn crossing of the Bosphorus, where thermals collapse earliest in the afternoon.

Unlike songbirds, raptors do not inherit the migration route genetically in a vector-and-distance sense. Young steppe eagles appear to rely heavily on following experienced conspecifics during their first southbound migration, a social inheritance confirmed by Perrins et al. (2018) in a Turkish-release study. Juveniles released outside the migration period followed inexperienced paths and suffered high mortality.

Natal philopatry and gene flow

Despite the long migrations, most populations show strong natal philopatry — individuals return to within 50 km of their hatch site (Newton 1979). This creates a population-genetic texture where mtDNA haplotypes are geographically structured along the breeding range despite the 12 000 km annual round-trip.