Case study: Visual tracking of Roseate Terns from Rockabill in 2018

ECON have undertaken a number of projects targeted towards research and conservation of seabirds. These include tagging studies, developing novel techniques to follow seabirds at sea, and establishing analytical approaches to defining Marine Protected Areas for breeding seabirds.

This case study describes the visual tracking of Roseate Terns from Rockabill, funded through the Roseate Tern LIFE Recovery Project, targeted at improving the conservation prospects of the species throughout the UK and Ireland.

Visual tracking involves following birds at sea in a RIB, revealing offshore foraging behaviour and identification of key offshore foraging areas.

During the 2018 breeding season 89 birds were tracked at sea. The average duration of a foraging trip was 39.6 minutes, with birds travelling an average of 21.12 km over the course of a foraging trip.

When following birds, it is often possible to broadly identify prey items, and to estimate the length of the item by measuring it against the birds bill length. Analysis of prey length data showed that Roseate terns typically select the 'best' item to carry back to the colony to feed chicks.

Tracklines showed a temporal shift in foraging behaviour, with birds foraging inshore along the coastline early season and offshore later on.

Roseate terns regularly visited multispecies foraging aggregations, initiated by auks driving prey to the surface. However, they appeared to have low foraging success in aggregations and generally left quickly. This is not surprising as multi species foraging aggregations build and end in gulls and Gannets, and terns are unable to compete with these bigger birds.

Observations of prey delivered to nests and comparison of mean chick provisioning rate (prey items delivered per hour) showed that provisioning rates were low in comparisons with other sites. Thus it seems likely that the observed mean trip length of just under 40km may be at the upper limit for the species.

Since Common Terns also nest on Rockabill, it is likely that the two species compete for shared resources. Although Common terns dominate around the colony, along the coasts, and perhaps in aggregations, Roseate terns have superior dive depth. Further tracking work focussed on both species would further understanding on species interactions and resource partitioning at this important colony.


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