First tracking of declining Caspian terns Hydroprogne caspia breeding in the Baltic Sea reveals high migratory dispersion and disjunct annual ranges as obstacles to effective conservation

Abstract

The conservation of migratory species poses significant challenges that may be countered by detailed knowledge about the sites used by migrants throughout the annual cycle. We present the first GPS-tracking data on the migration of declining Caspian terns Hydroprogne caspia breeding in the Baltic Sea. For 39 Caspian terns from colonies along a latitudinal gradient from 57 to 65°N, we identified key migratory routes, stopovers and wintering areas. In autumn these seabirds migrated using coastal and freshwater stopovers along six routes to reach their wintering areas across the Sahel, the Nile River Basin and the southern Iberian Peninsula. In spring, adults returned to the breeding grounds in the Baltic using a time optimizing strategy by reducing time at stopover by 78%, whereas most subadults remained sedentary and some performed only partial return migrations. Of the stopover sites used in both seasons, 58% are protected and have a reported management plan. Conservation strategies in wintering areas, stopover sites that are not protected or had not been previously recognized, and the inclusion of the species in important migratory flyways across Europe and Africa will be important to prevent further population declines of a species that depends on aquatic habitats.

Foraging on the wing for fish while migrating over changing landscapes: traveling behaviors vary with available aquatic habitat for Caspian terns

“…The exception was when adults also had very high diurnal flight speeds in spring in the northbound crossing of the Sahara with favorable tailwinds at an average of 3800 m above the ground. Notably, when crossing the Sahara Desert and the Mediterranean Sea the birds adopted a “sprinting” migration with increased travel speeds, flight heights and daily flight hours, resulting in a mixed migratory behavior that varies with available habitat [ 26 ] and combines foraging along the route with stopovers in freshwater and coastal ecosystems [ 34 ]. Mixed strategies that include both stopovers and fly-and-forage have been previously recognized for a handful of species including ospreys [ 26 , 27 ], Eleonora’s falcons [ 61 ], lesser black-backed gulls [ 30 ], Cory’s shearwaters [ 62 ], common swifts [ 63 , 64 ] and bank swallows [ 65 ].…”

“…Complete migratory tracks were obtained from solar powered 15–20 g GPS-GSM/3G devices (Ornitela) mounted with Teflon leg harnesses on 39 Caspian terns (12 adults, 27 juveniles) from 5 breeding colonies in the Baltic Sea in Sweden, as explained in [ 34 ] and available in [ 37 ]. The tracks had a high temporal resolution (recordings occurred every 5 min to maximum 2 h depending on battery), and were processed to exclude low-quality points and resampled into 30-min intervals using the package amt in R [ 38 ].…”

“…Migratory tracks began when birds left either their breeding or wintering grounds for autumn and spring migration, respectively, and ended in the last “traveling” day before reaching the areas they occupied during the corresponding stationary months. Stopovers were defined as the areas where birds remained during days when they did not advance more than 35 km between last daily relocations [ 22 , 30 ] because this threshold was over the average daily distance covered when birds were more sedentary (months January through March) and close to the median value separating the two modes when individual daily speeds deviated from an unimodal distribution (34.96 km/day, see [ 34 ]). Traveling segments were obtained from the tracks by excluding identified stopovers, so a traveling segment is the track of a bird when it is covering migration distance between stopover periods (see Additional file 1 : Fig.…”

“…In autumn, Caspian terns leave breeding colonies and remain in nearby staging areas in the Baltic Sea. For this season we considered such periods to represent initial fuelling before first, longer distance migratory flights [as in 34 ]. However, fuelling before migratory flight in spring occurs within wintering areas and so was not directly available from tracking data.…”

“…They are good fliers and forage with their heads directed downwards to visually detect their prey [ 36 ], making them possible fly-and-forage migrants. However, Caspian terns migrate an average of 7,111 km in autumn and 6,290 km in spring between their breeding grounds in the Baltic Sea and their wintering grounds in the Iberian Peninsula, Nile River Delta and sub-Saharan Africa [ 34 ]. Since their migration includes crossing continental Europe, and for some individuals also the Mediterranean Sea and Sahara Desert, behaviors related to a fly-and-forage strategy may be limited by the substantial variation in the availability of aquatic habitats along the route.…”