Abstract
Group travel is a familiar phenomenon among birds but the causes of this mode of movement are often unclear. For example, flocking flight may reduce flight costs, enhance predator avoidance or increase foraging efficiency. In addition, naive individuals may also follow older, more experienced conspecifics as a learning strategy. However, younger birds may be slower than adults so biomechanical and social effects on flock structure may be difficult to separate. Gannets are wide‐ranging (100s–1000s km) colonial seabirds that often travel in V or echelon‐shaped flocks. Tracking suggests that breeding gannets use memory to return repeatedly to prey patches 10s–100s km wide but it is unclear how these are initially discovered. Public information gained at the colony or by following conspecifics has been hypothesised to play a role, especially during early life. Here, we address two hypotheses: 1) flocking reduces flight costs and 2) young gannets follow older ones in order to locate prey. To do so, we recorded flocks of northern gannets commuting to and from a large colony and passing locations offshore and used a biomechanical model to test for age differences in flight speeds. Consistent with the aerodynamic hypothesis, returning flocks were significantly larger than departing flocks, while, consistent with the information gathering hypothesis, immatures travelled in flocks more frequently than adults and these flocks were more likely to be led by adults than expected by chance. Immatures did not systematically occupy the last position in flocks and had similar theoretical airspeeds to adults, making it unlikely that they follow, rather than lead, for biomechanical reasons. We therefore conclude that while gannets are likely to travel in flocks in part to reduce flight costs, the positions of immatures in those flocks may result in a flow of information from adults to immatures, potentially leading to social learning.
Highlights
Many birds travel in groups, but this subjects individuals to greater competition than lone travel
We considered the possibility that immatures are slower than adults indirectly, by testing whether immatures occupy the last position in mixed-age flocks and whether morphology differs sufficiently between adults and immatures to result in differing theoretical flight speeds
Wind speed at Leuchars, the nearest coastal weather station to Bass Rock (BR) averaged 4.7 m s–1 during the data collection period, which is similar to the June–August, long-term, average of 4.2 m s–1 (UK Meteorological Office 2018)
Summary
Many birds travel in groups, but this subjects individuals to greater competition than lone travel. By travelling in flocks, geese, swans and Nordic Society Oikos. It is becoming apparent that it may have additional social functions (Herbert-Read et al 2013). Naïve common ravens Corvus corax follow conspecifics that have recently discovered the locations of carrion (Marzluff et al 1996) and immature whooping cranes Grus americana learn migratory routes by following older conspecifics (Mueller et al 2013). We ask whether the structure of gannet flocks provides any evidence to support two not necessarily mutually exclusive hypotheses: firstly, that immature gannets travel in groups in order to reduce flight costs and secondly that they do so to locate prey
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