How tropical storms can help young seabirds

The destructive power of hurricanes are infamous but new research shows tropical storms could be a helping hand for young seabirds. Blogging today, the senior research fellow behind the study, Dr. Malcolm Nicoll, tells us more.  

Tropical cyclones (also known as hurricanes and typhoons) are renowned for their destructive nature when they hit land, destroying coastal urban areas and displacing thousands of people.

Cyclones are also known to affect wildlife in coastal environments and in particular seabirds at their breeding colonies causing a reduction in breeding success, primarily through the loss of eggs and chicks.  

However, cyclones originate and spend most of their lifetime in the marine environment where their impacts on seabird survival are poorly understood with the exception of infrequent seabird ‘wrecks’, whereby adult seabirds are caught between land and a cyclone and driven inshore.

Exploring the impacts of cyclones on seabirds’ survival is challenging because we need location data to demonstrate where and when seabirds are exposed to cyclones and long-term ecological data on survival to then measure any impact of exposure.

While remotely-sensed satellite data on cyclone tracks are readily available, corresponding location data for seabirds is not and in many seabird species these challenges are intensified as seabirds will spend a certain amount of time away from their breeding colony each year at-sea in unknown locations.

Since 1994 the Mauritian Wildlife Foundation and the National Parks and Conservation Service (Government of Mauritius) have been conducting a ringing and research programme for seabirds on Round Island, a nature reserve 23 km off the North coast of Mauritius in the tropical Indian Ocean.

The ringing programme involves the marking of seabirds, usually as chicks in the nest, with uniquely numbered metal leg rings, which allow the birds to be individually identifiable when caught later in life.

One of the focal species of this study is the Round Island Petrel, a medium-sized seabird that can live for over 40 years.

These particular petrels typically spend six months of each year at the breeding colony where they nest under rock ledges, clusters of boulders and in the native tussock grass and six months at-sea during their non-breeding migration.

Each year, regular surveys of the breeding colony are conducted and petrels are caught and identified from uniquely numbered metal leg rings.

Both adult and chick petrels without rings are ringed and around 3,300 petrels have been ringed to date. This 20 year data set comprised of ringing and recapture events (where a petrel with a ring is recaptured and identified) allow us to estimate the year to year survival of adult petrels and also the survival rate of young petrels during their first year after leaving the breeding colony.

Since 2009, we have been involved with the petrel monitoring programme and have established a tracking study to identify where Round Island petrels go during their non-breeding migration.

Between 2009 and 2011, 220 tracking devices were deployed on adult petrels and 24 on petrel chicks that were very close to leaving the breeding colony for the first time.

The tracking devices are miniature light loggers known as geolocators, which are mounted on plastic rings and deployed on the birds’ legs.

Geolocators have to be recovered from the bird before data can be downloaded, last for up to five years and record changes in ambient light levels such as sunrise and sunset events, which are then used to estimate locations twice a day using a process known as geolocation.

By the end of 2015, 83% of tracking devices had been recovered from adults and 50% from chicks and we established, for the first time, the year-round distribution of 120 adult petrels and six chicks during their first year at sea.

When compared with cyclone tracking data our location data for adult and juvenile petrels indicated that both juvenile and adult petrels were exposed to cyclones not just in the vicinity of Round Island during the breeding season, but also in the Arabian Sea and Bay of Bengal across the Northern Indian Ocean during their non-breeding migration.

By combining annual petrel survival data with corresponding regional cyclone metrics we were surprised to find that tropical cyclones can have contrasting effects on juvenile survival, depending on when and where these weather systems are encountered by petrels during their first year.

Intense cyclones in the vicinity of the breeding colony can reduce the survival of naïve, young petrels leaving the colony for the first time, but high levels of cyclone activity in the Northern Indian Ocean actually appear to significantly improve survival over their first year.

Regarding the latter, we suspect that there’s a link with strong localised cyclone winds forcing more nutrient-rich waters to the surface, and thus additional food sources for the young petrels at a time in their lives when they are still learning about their environment and where best to find food.

We are hoping that further research will shed more light on these intriguing and somewhat counter-intuitive findings.

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The research has come from key collaboration between ZSL, the National Parks & Conservation Service and the Mauritian Wildlife Foundation.