Changes in behaviour drive inter-annual variability in the at-sea distribution of northern gannets

Abstract

The at-sea distribution of seabirds primarily depends on the distance from their breeding colony, and the abundance, distribution and predictability of their prey, which are subject to strong spatial and temporal variation. Many seabirds have developed flexible foraging strategies to deal with this variation, such as increasing their foraging effort or switching to more predictable, less energy dense, prey, in poor conditions. These responses may vary both within and between individuals, and understanding this variability is vital to predict the population-level impacts of spatially explicit environmental disturbances, such as offshore windfarms. We conducted a multi-year tracking study in order to investigate the inter-annual variation in the foraging behaviour and location of a population of northern gannets breeding on Alderney in the English Channel. To do so, we investigated the link between individual-level behaviour and population-level behaviour. We found that a sample of gannets tracked in 2015 had longer trip durations, travelled further from the colony and had larger core foraging areas and home range areas than gannets tracked in previous years. This inter-annual variation may be associated with oceanographic conditions indexed by the North Atlantic Oscillation (NAO). Our findings suggest that this inter-annual variation was driven by individuals visiting larger areas in all of their trips rather than individuals diversifying to visit more, distinct areas. These findings suggest that, for gannets at least, if prey becomes less abundant or more widely distributed, more individuals may be required to forage further from the colony, thus increasing their likelihood of encountering pressures from spatially explicit anthropogenic disturbances.

Personality predicts foraging site fidelity and trip repeatability in a marine predator

“…Owing to distinctly different foraging behaviour between incubation and chick-rearing periods (Robertson et al, 2014; Table 1), data were analysed separately by breeding stage. During data processing, we removed points within a 300 m buffer of each colony (based on the frequency histogram of point distance to the colony) and defined foraging trips as periods longer than 1 hr spent outside this buffer (based on the frequency histogram of trip durations; Warwick-Evans et al, 2016). Trips longer than 1 hr may still include trips carried out for purposes besides foraging, such as bathing.…”

Environmental heterogeneity amplifies behavioural response to a temporal cycle

“…GPS data were not interpolated. To eliminate departures from the colony because of disturbance (Collins et al 2016, Warwick-Evans et al 2016b), we excluded points closer than 300 m to the colony, and attributed sequential points to a foraging trip if the total trip duration was over 14 min (based on a frequency distribution of trip duration; Warwick-Evans et al 2016b). Hereafter we refer to fixes recorded by GPS loggers as locations used by kittiwakes.…”

Sample size required to characterize area use of tracked seabirds

“…The extent of inter‐ and intra‐annual spatial variation in animal movements is well known for many central‐place foragers (Sharples et al , Shackleton et al ), including seabirds (Hamer et al , Warwick‐Evans et al ). The extent of annual coverage in tracking studies could therefore influence final sampling protocols recommended.…”

“…Coefficients of a-c denote the shape of the curve relationship in each model. including seabirds (Hamer et al 2007, Warwick-Evans et al 2016. The extent of annual coverage in tracking studies could therefore influence final sampling protocols recommended.…”