Young Birds Switch but Old Birds Lead: How Barnacle Geese Adjust Migratory Habits to Environmental Change

Thomas Oudman,Ingunn Tombre,Graeme Ruxton,Paul Shimmings,Kevin Laland,Jouke Prop

doi:10.3389/fevo.2019.00502

Abstract

Long-distance migratory animals must contend with global climate change, but they differ greatly in whether and how they adjust. Species that socially learn their migration routes may have an advantage in this process compared to other species, as learned changes that are passed on to the next generation can speed up adjustment. However, evidence from the wild that social learning helps migrants adjust to environmental change is absent. Here, we study the behavioural processes by which barnacle geese (Branta leucopsis) adjust spring-staging site choice along the Norwegian coast, which appears to be a response to climate change and population growth. We compared individual-based models to an empirical description of geese colonizing a new staging site in the 1990s. The data included 43 years of estimated annual food conditions and goose numbers at both staging sites (1975–2017), as well as annual age-dependent switching events between the two staging sites from one year to the next (2000–2017). Using Approximate Bayesian Computation, we assessed the relative likelihood of models with different ‘decision rules’, which define how individuals choose a staging site. In the best performing model, individuals travelled in groups and staging site choice was made by the oldest group member. Groups normally returned to the same staging site each year, but exhibited a higher probability of switching staging site in years with larger numbers of geese at the staging site. The decision did not depend on food availability in the current year. Switching rates between staging sites decreased with age, which was best explained by a higher probability of switching between groups by younger geese, and not by young geese being more responsive to current conditions. We found no evidence that the experienced foraging conditions in previous years affected staging site choice. Our findings demonstrate that copying behaviour and density-dependent group decisions explain how geese adjust their migratory habits rapidly in response to changes in food availability and competition. We conclude that considering social processes can be essential to understand how migratory animals respond to changing environments.

Highlights

The choices that animals make in response to their environment have typically been shaped by evolution, and are expected to maximize the animal’s survival and reproduction
It remains largely unknown how migratory animals combine current and previous individual experiences with social learning to make decisions, and whether this combination helps them to adjust their migrations to environmental change
Simulations resembled the empirical data best when geese were assumed to travel in small groups that are led by the oldest individuals, and when young geese switched more between groups in subsequent years than did older individuals (Table 2, Figure 5)

Summary

Introduction

The choices that animals make in response to their environment have typically been shaped by evolution, and are expected to maximize the animal’s survival and reproduction. Animals must deal with uncertainty in the consequences of their decisions To understand those decisions, it is necessary to know which environmental factors individuals use to inform their decision, and how they integrate those factors to make the decision (i.e., their “decision rules”; Bauer et al, 2011; Budaev et al, 2019). Recent semi-natural experiments suggest that animal populations can accumulate improvements of migratory routes over several generations by combining individual learning with social learning (Sasaki and Biro, 2017; Jesmer et al, 2018), but evidence from natural populations is lacking It remains largely unknown how migratory animals combine current and previous individual experiences with social learning to make decisions, and whether this combination helps them to adjust their migrations to environmental change

Methods

Results

Conclusion