Migratory connectivity is a term used to describe the relationship between populations of animals (especially birds) and geographic locations at different time points during the year. North American migratory birds typically breed in northern latitudes during the summer and overwinter in southern latitudes. In general, populations of a migratory species will take one of four major migratory routes between breeding and wintering grounds. However, not all populations of a species take the same routes. For example, Swainson’s thrush (Catharus ustulatus) take two different non-overlapping migratory routes to and from northern and southern latitudes. Coastal populations travel along the Pacific coast while continental populations migrate along eastern routes (Reugg and Smith 2002).
Migratory connectivity studies are particularly useful for developing conservation strategies, tracking bird-borne diseases, and for understanding population genetics, gene flow, and speciation. Advances in genetic technologies and stable-isotope analysis are increasing our ability to detect patterns of migratory connectivity. Traditional methods of banding and recapture provide limited details about the migratory patterns of a species of bird because few banded individuals are ever captured twice. However, genetic techniques can provide population-level data about breeding and wintering grounds and the migratory routes connecting the two.
Genetic analyses take advantage of population-specific variations in the DNA of bird species. Members of the same population are more closely related and, therefore, have more genetic markers in common than individuals from two different populations. If a species of bird maintains several different breeding populations, then the genetic differences between populations can be used to connect individual birds back to a specific population. Thus, capturing individual birds at any point during the year can provide information about where a specific population breeds, and overwinters, and which migratory routes it follows.
Stable-isotopic analysis can also be used to help identify the breeding grounds used by individual birds. There are known longitudinal and latitudinal concentration gradients for stable isotopes. Animals feeding at a location will ingest food with a specific concentration of stable isotope. These isotopes will then be incorporated into the physical structures of the bird that are developed at that location (e.g., feathers). Thus one can estimate the location where feathers are developed by the concentrations of stable isotope present in those feathers. If a bird is captured on its wintering grounds, the regional location of the breeding grounds from the previous summer can be identified.
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This image of an AFLP (Amplified Fragment Length Polymorphism) assay is an example of a genetic fingerprinting method that is used to assess migratory connectivity.
This image (from http://biocycle.atmos.colostate.edu/sib.htm) is an example of a stable isotope concentration gradient map.
Avian Influenza A and Migratory Birds
Bird flu reached the public's attention in 1996 and 1997 after the H5N1 outbreaks of highly virulent avian influenza A virus (AIV) in Asia, Africa, and Europe. Researchers have known for years that low pathogenic AIV is prevalent in birds. In fact, AIV has been isolated from dozens of species. Despite this knowledge, little is actually known about the prevalence of AIV in birds or how AIV is transmitted through avian populations via migratory routes (Stallknecht and Shane 1988; Webster et al.1992). The threat of H5N1 in migratory species has made this field of study particularly relevant.
Several lineages of H5N1 have been found in poultry, and data suggests that H5N1 evolved in several hosts. Historically, the first H5 avian subtype was identified from wild terns in South Africa in 1961 (A/tern/South Africa/61 [H5N3]), and the precursor of the current highly pathogenic H5N1 lineage was sampled in China around 1996. Since then, numerous genotypes of H5N1 have circulated throughout Southeast Asia via waterfowl and poultry (Webster et al. 2006 ).
The H5N1 viruses probably emerged in Asia in 2001 and 2002 from the reassortment and evolution of many different genotypes. These putative ancestor genotypes were detected in poultry markets and farms in Hong Kong (Li et al. 2004; Chen et al. 2005; Liu et al. 2005). In August 2005, highly pathogenic H5N1 killed thousands of migratory waterfowl around Lake Qinghai in western China (Liu et al. 2005 ) and an additional 89 migratory birds in Mongolia. By February of 2006, H5N1 was isolated from 21 wild bird species (mostly in Asia) and 23 captive species. An additional 12 species have been experimentally infected.
Although major outbreaks of H5N1 occurred independent of typical migration cycles, migratory birds have been strongly implicated in the carriage and dissemination of the H5N1 strain. To date several passerine species have been identified as potential conduits for the introduction of AIV subtypes, including H5N1, from Asia to North America. For example, Yellow Wagtails commonly breed in western Alaska and winter near agricultural fields in southeast China and Indonesia (Badyaev et al. 1998). Some North American Nearctic-Neotropical migrant land birds share breeding and/or migration stopover habitats with Yellow Wagtails and other Eurasian migratory species that could potentially carry a variant of H5N1.
The information regarding the dissemination of AIV along migratory routes, and between birds and humans, will be critical if H5N1 enters populations of North American migratory birds. Some species of non-migratory birds (e.g., Sparrows, Starlings, Blackbirds, and nest-parasitizing Cowbirds) share the same habitat as poultry and domesticated birds. If these non-migratory birds contract H5N1 from carrier migratory birds, then they could spread the strain to poultry. Poultry are very susceptible to H5N1 and could, in turn, infect humans associated with the poultry trade. Thus, non-migratory birds and poultry would act as vectors for the transmission of AIV between migratory birds and humans.
A compounding factor of AIV detection in wild birds is that it tends to asymptomatically infect songbirds. This means that an H5N1 virus could potentially spread and evolve in bird populations and along migratory routes with little warning or initial indication. Widespread monitoring of North American land birds is paramount. Not only is it important for the detection of H5N1, but also for providing details about the ecology of endemic low-pathogenic AIV. Ecological information could then be used to generate a predictive model of a potential H5N1 outbreak.
Badyaev, A. V., B. Kessel, and D. D. Gibson. 1998. Yellow Wagtail (Motacilla flava). In A. Poole and F. Gill (Eds.) The Birds of North America, No. 382. The Birds of North America, Inc., Philadelphia, PA.
Chen, H., G. J. D. Smith, S. Y. Zhang, K. Qin, J. Wang, K. S. Li, R. G. Webster, J. S. M. Peiris, and Y. Guan. 2005. Avian flu: H5N1 virus outbreak in migratory waterfowl. Nature 436: 191–2.
Li, K. S., Y. Guan, J. Wang, G. J. Smith, K. M. Xu, L. Duan, A. P. Rahardjo, P. Puthavathana, C. Buranathai, T. D. Nguyen, A. T. S. Estoepangestie, A. Chaisingh, P. Auewarakul, H. T. Long, N. T. H. Hanh, R. J. Webby, L. L. M. Poon, H. Chen, K. F. Shortridge, K. Y. Yuen, R. G. Webster, and J. S. M. Peiris. 2004. Genesis of a highly pathogenic and potentially pandemic H5N1 influenza virus in eastern Asia. Nature 430: 209–213.
Liu, J., H. Xiao, F. Lei, Q. Zhu, K. Qin, X. W. Zhang, X. L. Zhang, D. Zhao, G. Wang, Y. Feng, J. Ma, W. Lui, J. Wang, and G. F. Gao. 2005. Highly pathogenic H5N1 influenza virus infection in migratory birds. Science 309: 1206.
Ruegg, K. and T. B. Smith. 2002. Not as the crow flies: a historical explanation for circuitous migration in Swainson's thrush (Catharus ustulatus). Proceedings of the Royal Society, London 269: 1375-1381.
Stallknecht, D. E. S. and M. Shane. 1998. Host range of avian influenza virus in free-living birds. Veterinary Research Communications 12: 125–141.
Webster, R. G , W. J. Bean, O. T. Gorman, T. M. Chambers, and Y. Kawaoka. 1992. Evolution and ecology of influenza A viruses. Microbiology Review 56: 152-179.
Webster, R. G., M. Peiris, H. Chen, and Y. Guan. 2006. H5N1 Outbreaks and Enzootic Influenza. Emerging Infectious Diseases 12: 3-8.