Are non-breeding animals “super-spreaders” of disease?

Juliet Lamb, Jeremy Tornos, Romain Dedet, Hubert Gantelet, Nicolas Keck, Juliette Baron, Marine Bely, Augustin Clessin, Aline Flechet, Amandine Gamble, Thierry Boulinier

This is a plain language summary of a Functional Ecology research article which can be found here.

Most wildlife tracking studies focus on breeders, which are easy to observe and capture at their breeding sites. In any given year, however, many individuals do not breed due to age, social status, resource limitations, or other factors. Since non-breeders aren’t tied to specific locations, they may be free to wander more widely than breeders. At the same time, they are more difficult to capture and track than breeders, making them rare or absent in movement studies.

One way to address this gap in movement studies could be to capture animals at sites used by both breeders and non-breeders. For instance, some seabirds attending breeding colonies visit loafing sites or “clubs,” where breeding and non-breeding birds congregate when they’re not foraging or tending young. To test whether sampling at club sites changed estimates of movement, we captured brown skuas at both nest sites and clubs on Amsterdam Island, a remote breeding colony in the Indian Ocean. We fitted skuas with GPS transmitters and tested them for the bacteria that causes recurring outbreaks of avian cholera. We wanted to see whether non-breeders moved greater distances than breeders, and to assess whether non-breeders might be reservoirs and spreaders of avian cholera among seabirds on Amsterdam Island.

A brown skua on Amsterdam Island wearing a GPS transmitter (Photo credit: Marine Bely)

We found that non-breeding skuas had larger home ranges than breeders, overlapped more with other seabirds, and spent more time flying and foraging and less time resting. Although most skuas were exposed to avian cholera, only non-breeders had both active cholera infections and large enough home ranges to transmit bacteria among different seabird breeding areas. Some non-breeders repeatedly returned to the same loafing site, while others did not use consistent locations. This high degree of variability among non-breeders may reflect the variety of different reasons for which individuals forego breeding.

Our work shows that including non-breeders in movement studies can increase estimates of range and variability, with important conservation consequences. For example, planned eradications of introduced rats (a key prey item for skuas) could affect the numbers, foraging distances, and pathogen transmission potential of non-breeders. In addition, recent outbreaks of highly pathogenic avian influenza have caused extensive seabird mortality, and the wide-ranging movements of non-breeders could help spread the disease among colonies. Identifying non-breeders who might act as “super-spreaders” could help to predict future outbreaks and design targeted management strategies.

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