Abstract
BackgroundWinter numbers of the northwest European population of Bewick’s Swans (Cygnus columbianus bewickii) declined recently by c. 40%. During the same period, numbers of two sympatric and ecologically-similar congeners, the Mute Swan (Cygnus olor) and Whooper Swan (Cygnus cygnus) showed increases or stability. It has been suggested that these opposing population trends could have a causal relationship, as Mute and Whooper Swans are larger and competitively dominant to Bewick’s Swans in foraging situations. If so, effects of competition of Mute and Whooper Swans on Bewick’s Swans should be detectable as measurable impacts on behaviour and energetics.MethodsHere, we studied the diurnal behaviour and energetics of 1083 focal adults and first-winter juveniles (“cygnets”) of the three swan species on their winter grounds in eastern England. We analysed video recordings to derive time-activity budgets and these, together with estimates of energy gain and expenditure, were analysed to determine whether individual Bewick’s Swans altered the time spent on key behaviours when sharing feeding habitat with other swan species, and any consequences for their energy expenditure and net energy gain.ResultsAll three swan species spent a small proportion of their total time (0.011) on aggressive interactions, and these were predominantly intraspecific (≥ 0.714). Mixed-effects models indicated that sharing feeding habitat with higher densities of Mute and Whooper Swans increased the likelihood of engaging in aggression for cygnet Bewick’s Swans, but not for adults. Higher levels of interspecific competition decreased the time spent by Bewick’s Swan cygnets on foraging, whilst adults showed the opposite pattern. When among low densities of conspecifics (< c. 200 individuals/km2), individual Bewick’s Swans spent more time on vigilance in the presence of higher densities of Mute and Whooper Swans, whilst individuals within higher density Bewick’s Swan flocks showed the opposite pattern. Crucially, we found no evidence that greater numbers of interspecific competitors affected the net energy gain of either adult or cygnet Bewick’s Swans.ConclusionsWe found no evidence that Bewick’s Swan net energy gain was affected by sharing agricultural feeding habitat with larger congeners during winter. This was despite some impacts on the aggression, foraging and vigilance behaviours of Bewick’s Swans, especially among cygnets. It is unlikely therefore that competition between Bewick’s Swans and either Mute or Whooper Swans at arable sites in winter has contributed to the observed decline in Bewick’s Swan numbers. Further research is needed, however, to test for competition in other parts of the flyway, including migratory stopover sites and breeding areas.
Highlights
Winter numbers of the northwest European population of Bewick’s Swans (Cygnus columbianus bewickii) declined recently by c. 40%
The observed decline in Bewick’s Swan numbers was in contrast to many other populations of large herbivorous waterbirds in Europe, which have shown stable or increasing population trends since the Bewick’s Swan population peaked in the mid-1990s (Rees et al 2019). These include two congeners of the Bewick’s Swan that are native to Europe: the Mute Swan (Cygnus olor), which is largely sedentary throughout northwest Europe, and the two populations of migratory Whooper Swan (Cygnus cygnus), one of which breeds in Iceland and winters in the UK and Ireland, while the other breeds in northwest Russia and Fennoscandia, and winters in mainland northwest Europe (Rees et al 2019)
We found that interactions between focal age class and Bewick’s Swan density had variance inflation factor (VIF) values of < 5.0 (Dormann et al 2013), and so models containing this interaction were removed from our sets of candidate models for each of the four response variables, and were not considered further, so that multicollinearity did not influence the efficiency and reliability of parameter estimation for our models (Dormann et al 2013)
Summary
Winter numbers of the northwest European population of Bewick’s Swans (Cygnus columbianus bewickii) declined recently by c. 40%. The observed decline in Bewick’s Swan numbers was in contrast to many other populations of large herbivorous waterbirds in Europe, which have shown stable or increasing population trends since the Bewick’s Swan population peaked in the mid-1990s (Rees et al 2019) These include two congeners of the Bewick’s Swan that are native to Europe: the Mute Swan (Cygnus olor), which is largely sedentary throughout northwest Europe, and the two populations of migratory Whooper Swan (Cygnus cygnus), one of which breeds in Iceland and winters in the UK and Ireland, while the other breeds in northwest Russia and Fennoscandia, and winters in mainland northwest Europe (Rees et al 2019). Whooper Swan numbers have more than doubled since 1995 (Hall et al 2016; Laubek et al 2019)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
