Density-dependent competition for food influences the foraging behaviour and population dynamics of colonial animals. Tracking technology can provide new insights in how these patterns vary among populations, and collaboration between research groups studying the same species across different parts of their range allows us to compare results across breeding sites.
The Northern Gannet is one of the best-known and most tracked seabird species. A recent open access study used the Seabird Tracking Database and published papers to gather GPS data for 1,118 gannets from 21 colonies during the crucial chick-rearing period.
Figure 1 from Clark et al. 2024: All 54 known Northern Gannet colonies with circles proportional to colony size.
The results supported previous work showing that the distance and duration of foraging trips are influenced by the size of the colony, which varied from ~100 to ~60,000 breeding pairs. Gannets from larger colonies spent more time on their foraging trips and travelled further to find food. This pattern suggests that competition for food increases with colony size, pushing birds to venture farther afield.
Interestingly, the research also highlighted a latitudinal variation in foraging behaviour. Gannets from northern colonies had shorter foraging trips than their southern counterparts. This indicates that food availability is higher nearer the poles, reducing competition and potentially allowing for rapid population growth in these areas. This provides a potential explanation for the species’ poleward shift and poor breeding success in some southern colonies, which may relate to climate change.
For some colonies, tracking work had been done in up to 10 different years. This showed that environmental variability is also likely to play a significant role, with big differences between years observed within the same colony.
Figure 2 from Clark et al. 2024: Mean foraging trip duration & maximum distance from the colony for 21 gannet colonies in relation to colony size and latitude. Black lines show the prediction from a finite population corrected Linear Model ± 95% confidence intervals (grey ribbon) using the mean value of latitude to predict the effect of square-root (√) colony size and vice versa. To visually correct for √colony size, we subtracted the √colony size effect (grey dashed line) from each datapoint (grey circle) and then added the √colony size effect for the mean √colony size (black circle).
As the climate continues to change, understanding the dynamics of prey availability and the impact on foraging behaviour can be crucial for seabird conservation. Measuring foraging effort could provide an early warning that colonies may be in trouble before there is an impact on population size. This could inform conservation prioritisation and help us understand the impact of events such as the recent outbreak of Highly Pathogenic Avian Influenza. While directly addressing the impact of climate change on marine systems is challenging, reducing the impact of other threats, such as bycatch, overfishing, invasive species and pollution can make populations more resilient.