Spatial and temporal variation in foraging of breeding red‐throated divers

Abstract

Differing environmental conditions can have profound effects on many behaviours in animals, especially where species have large geographic ranges. Seasonal changes or progression through life history stages impose differential constraints, leading to changes in behaviours. Furthermore, species which show flexibility in behaviours, may have a higher capacity to adapt to anthropogenic‐induced changes to their environment. The red‐throated diver (RTD) is an aquatic bird, that is able to forage in both freshwater and marine environments, though little else is known about its behaviours and its capacity to adapt to different environmental conditions. Here, we use time‐depth recorders and saltwater immersion loggers to examine the foraging behaviour of RTDs from three regions across northwest Europe. We found that in the breeding season, birds from two regions (Iceland and Scotland) foraged in the marine environment, while birds from Finland, foraged predominantly in freshwater. Most of the differences in diving characteristics were at least partly explained by differences in foraging habitat. Additionally, while time spent foraging did not change through the breeding season, dives generally became more pelagic and less benthic over the season, suggesting RTDs either switched prey or followed vertical prey movements, rather than increasing foraging effort. There was a preference for foraging in daylight over crepuscular hours, with a stronger effect at two of the three sites. Overall, we provide the first investigation of RTD foraging and diving behaviour from multiple geographic regions and demonstrate variation in foraging strategies in this generalist aquatic predator, most likely due to differences in their local environment.

Winter locations of red‐throated divers from geolocation and feather isotope signatures

“…The limited amount of movement observed in the Icelandic birds likely means molt is occurring in similar locations to the rest of the non‐breeding period locations. This population does still experience some seasonal change though, as they cease spending time in freshwater environments during the non‐breeding season (Duckworth et al, 2021). Our maps showed a westward movement for some birds during the study period (Figure 3a,b), from the northeast to northwest coast of Iceland.…”

“…All available locations from all individuals within the stated timeframes are used to generate estimates. RTDs from our study populations completed their breeding attempts by mid-late August (Duckworth et al, 2021), but locations shown are from the early winter period (22nd October-31st December) to late winter period (1st January-20th February) to exclude periods where there is still a noticeable impact on locations from the equinox periods. To further exclude any clearly anomalous data points, any points above 75° North were excluded, as often locations extracted when the GLS logger is heavily shaded are pushed to the northernmost degrees of latitude.…”

“…In Europe, birds from many populations can make large migratory flights, while some are thought to fly short distances or remain resident (Dorsch et al, 2019;Duckworth et al, 2020). Furthermore, year-round variation in habitat use can differ between individuals and populations, with RTD switching from a marine to a wholly freshwater distribution from the non-breeding to the breeding season, respectively (Duckworth et al, 2021). However, in Europe, we currently lack a comprehensive understanding of the year-round distributions of all populations.…”

Energetic synchrony throughout the non‐breeding season in common guillemots from four colonies

“…We used the mean rate of descent (1.38 ms −1 ) from Thaxter et al (2010) plus three standard deviations (SD: 0.16 ms −1 ) in order to account for 99% of the error observed and combined this with our sampling rate, resulting in a maximum depth change of 22.32 m within 12 s. Any depth records that exceeded this speed threshold were removed along with any records that exceeded the maximum measuring potential of the TDRs (200 m). All remaining records with a depth greater than 1 m were defined as diving (Duckworth et al 2020, 2021, Dunn et al 2020). Individual dives were characterised as excursions of depth greater than 1m and subsequent return to the surface.…”

“…Dives were categorised into foraging bouts, where more than two dives occurred with less than 180 s of surface time between them (Tremblay and Cherel 2000). To determine whether individuals were generally using a benthic or pelagic (mid‐water) foraging strategy, we calculated the proportion of dives that were within the intra‐depth zone (IDZ; Tremblay and Cherel 2000, Duckworth et al 2021). Higher proportions of dives within the IDZ indicate that a more benthic foraging strategy is being used, as dives are likely to have more similar depths when an individual is foraging benthically (Tremblay and Cherel 2000).…”