Immune system benefits could explain why birds migrate

Chuck Bednar for redOrbit.com – Your Universe Online
Bird migration, the regular seasonal movement of avian species along a flyway, often requires creatures to travel large distances from their breeding grounds and face fearsome predators and other potential dangers. So why do birds make these epic journeys?
Previous studies have suggested that the creatures benefit from the longer daylight hours, or that fewer predators await them in their new homes. However, experts from the Dutch center for bird migration and demographics (NIOO-KNAW Vogeltrekstation) and the University of Groningen have discovered a possible new explanation involving their immune systems.
The researchers investigated Dutch barnacle geese breeding at Spitsbergen, the largest island of the Svalbard archipelago in northern Norway, and compared them with birds of the same species that did not migrate by remained in the Netherlands throughout the entire year.
They found that the birds breeding at Spitsbergen appeared to invest significantly less energy in their immune systems, and especially the part responsible for general resistance to diseases. The study authors believe this might be due to the that the Netherlands are home to a greater number of pathogens than the regions located in the North.
Since those birds invest less energy in their immune system, they have more available for reproduction and changing their plumage, the researchers explained. Geese that breed in the Netherlands do not have this luxury, and in order to defend themselves from bird flu and other pathogens, their immune systems have to work four-times harder during the summer.
This is far from the first time that scientists from the University of Groningen have studies the barnacle goose. Experts there first started analyzing the Svalbard barnacle goose (Branta leucopsis) population at Spitsbergen in 1975, the university explained on its website. Since 1990, it began studying the population in Ny Ålesund, where barnacle geese has started breeding 10 years earlier.
Barnacle geese are herbivores with a relatively basic digestive tract, the university said, and they are selective foragers that require high-quality plants. Their flight capability brings them to the High Arctic in search of protein-rich spring growth, but that journey is often challenging due to the short length of the Arctic summer and the long migration distance.
“The preparation for successful breeding starts already during spring migration,” the institution said. There is limited food available upon the geese’s arrival at the breeding grounds, meaning that they need to lay eggs as quickly. Otherwise, their offspring would not have enough time to grow, fatten up and learn how to fly before the start of the autumn migration season.
“To compete with their conspecifics, experience and dominance are important. We observe how geese learn from previous experience and that the geese that win frequent occurring fights and interactions have the fastest growing goslings,” it added. “Successful individuals have a large chance of being successful in the next year too, without apparent costs of reproduction.”
They devised experiments in which they manipulated the geese, enlarging and decreasing broods by exchanging goslings between nests at hatching time. They found that enlarged broods became more dominant, had faster-growing goslings and heavier female parents that control broods or those that decreased in size.
Larger goslings survive also better to the next breeding season, and no impact was found on parental survival or future breeding success rates. It added that brood size is limited by body reserves during laying, as well weighing the advantages of hatching eggs early or laying an extra one, and that the benefits of brood size can help explain adoption and egg-dumping events.
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