Application Of Isoscapes In Migratory Animal Research

Birds

Following growth feathers are metabolically inert retaining information on geographical origins. However this method is based on underlying assumptions that can only be tested using birds of known origin. Many pelagic seabirds moult their feathers when at sea, behaviour with a high energy cost and high mortality rate. Understanding spatial and trophic ecology during this critical period is important for identifying demographic patterns. A fundamental requirement of implementing the use of stable isotopes is the development of a rescaling function linking the environment to measurements in the tissues of the species of interest, however for birds this has not been carried out – Hobson (2015) set out to investigate this using feathers from 104 individuals of 11 species of known source, along the strong precipitation isoscape of North America. A study by Haché et al (2012) aimed to assign birds to geographic origins using feather hydrogen isotope ratios. In North America where this study was carried out, gradients in the ratio of stable hydrogen isotopes in “amount-weighted, growing season mean precipitation form a largely latitude-sensitive isoscape that can be used to estimate the geographical origin of animals”.

Glew et al aimed to quantify species-specific foraging locations and individual trophic variability of the common guillemot, razorbill and Atlantic puffin during feather regrowth on the Isle of May, Scotland. The aims of Haché et al (2012) were to determine factors influencing intra- and inter- individual variability derived from Ovenbirds, associated with season, habitat type, year, feather type and age in New Brunswick, Canada. They predicted differences in years and ages as adults and juveniles moult at different times of the year, but also in feather types as the Ovenbird grows its flight feathers in the nest but its tail feathers prior to fledging, meaning both were exposed to different microclimates and energy conditions when grown, this also needs to be taken into account in analysis. The aims of Jaegar et al (2010) where to validate and quantify the relationship between predator C values and latitudes in the Southern Ocean using the albatross as this species forages widely from the Antarctic to subtropical waters during its incubation period.

Methods

Glew, et al combined two methods of geolocation in their study of auks in the North Sea: carbon and nitrogen isoscapes and bird-borne geolocation loggers. The nitrogen and carbon isoscapes for the North Sea developed using jellyfish by Trueman et al. (2017), were used to assign the seabirds to likely foraging areas. The bivariate normal probability function was used to estimate the likelihood that foraging areas of each individual are represented by specific North Sea carbon and nitrogen isoscapes. As auks are flightless during these periods, identified areas can be considered likely locations of shedding and feather regrowth. Jaegar et al (2010) set out to validate an isotopic gradient for higher trophic levels using the C and N plasma measurements of the albatross - a wide ranging oceanic predator. A combination of subsequent blood plasma isotopic signatures and a large number of satellite-tracks were used. -ResultsIndividual trophic position varied between the three birds in the Glew, et al (2018) study with guillemots consistently feeding at higher trophic levels. “Seasonal variations in isotopic compositions of primary producers and low trophic level consumers are probable”, but are likely attenuated during successive predator-prey interactions so that “higher trophic level consumers inherit isotopic compositions that reflect a seasonal average value”. Using these two methods of geolocation in tandem better constraints foraging areas, explores individual differences and flexibility in foraging strategy and provides vital information for seabird conservation as well as for marine spatial planning.

Many migratory bird species moult at their breeding grounds in North America prior to migration, this enables researchers to match feathers found elsewhere along their migratory route back to their origin. Jaegar et al (2010) identified a strong positive correlation between C and N values in the Southern Ocean and the plasma of wandering albatrosses. The data allowed Jaegar et al (2010) to estimate the carbon isoscape position of the main oceanic fronts in the area, and thus to delineate robust isoscapes of the main foraging zones for top predators. The spatial accuracy of the determination of consumers` foraging areas using their C values operates more at "a large geographical scale, that of water masses and fronts, than latitude per se. Thus, predators foraging within the same hydrological features would retain the same C values”.

“Interestingly, albatross N and C values are positively and linearly correlated, as are the C and N signatures of beaks from colossal squids”. The fact that these two very different organisms each with their own distinct isotopic signatures present the same relationship with similar slopes of the regression lines "suggests concomitant changes in baseline N and C values with latitudes”. This phenomenon is crucial and its importance and potential are clearly valuable in better interpretation and comparison of the nitrogen signature and predator foraging position both in terms of geolocation and trophic position, therefore highlighting the need for more information to quantify this latitudinal effect. The study of feathers is predominant in isoscape bird studies and has proven effective in a number of studies however Jaegar et al (2010) suggest that “body feathers do not allow an accurate comparison of device and isotopic tracking, because the precise timing of their synthesis during the extended seabird moulting period is not known”. As a result this study concludes by encouraging "the development of approaches that combine the tracking of seabirds and marine mammals using electronic devices with isotopic analysis of relevant tissues”.

Haché et al, (2012) concludes from their findings that using annual isoscapes, accounting for interannual precipitation during the breeding season and using age-specific correlation would prove advantageous. Current mechanisms responsible for the decoupling of Oxygen and Hydrogen isotopes is poorly known. Hobson (2015) recommends a much more vigorous effort to sample more individuals, taxa and precipitation gradients in order to increase the power of this method as a tool in animal migration research in particular the value of using water isotopes in migratory animal research.