Mapping the distribution of seabirds at sea is fundamental to understanding their
ecology and making informed decisions on their conservation. Until recently, estimates of
at-sea distributions were generally derived from boat-based visual surveys. Increasingly
however, seabird tracking is seen as an alternative but each has potential biases. To
compare distributions from the two methods, we carried out simultaneous boat-based
surveys and GPS tracking in the Minch, western Scotland, in June 2015. Over 8 days,
boat transect surveys covered 950 km, within a study area of ∼6,700 km2 centered on
the Shiant Islands, one of the main breeding centers of razorbills, and guillemots in the
UK. Simultaneously, we GPS-tracked chick-rearing guillemots (n = 17) and razorbills
(n = 31) from the Shiants. We modeled counts per unit area from boat surveys as
smooth functions of latitude and longitude, mapping estimated densities. We then used
kernel density estimation to map the utilization distributions of the GPS tracked birds.
These two distribution estimates corresponded well for razorbills but were lower for
guillemots. Both methods revealed areas of high use around the focal colony, but over
the wider region, differences emerged that were likely attributable to the influences
of neighboring colonies and the presence of non-breeding birds. The magnitude of
differences was linked to the relative sizes of these populations, being larger in guillemots.
Whilst boat surveys were necessarily restricted to the hours of daylight, GPS data
were obtained equally during day and night. For guillemots, there was little effect of
calculating separate night and day distributions from GPS records, but for razorbills
the daytime distribution matched boat-based distributions better. When GPS-based
distribution estimates were restricted to the exact times when boat surveys were carried
out, similarity with boat survey distributions decreased, probably due to reduced sample
sizes. Our results support the use of tracking data for defining seabird distributions
around tracked birds’ home colonies, but only when nearby colonies are neither large
nor numerous. Distributions of animals around isolated colonies can be determined using
GPS loggers but that of animals around aggregated colonies is best suited to at-sea
surveys or multi-colony tracking.