Staff Publications

In the migratory behaviour of the Barnacle Goose Branta leucopsis several changes have

occurred over the past few decades. Barnacle geese breeding in Russia have delayed the

commencement of spring migration with approximately one month since the 1980s,

new populations have emerged in former stopover areas in the Baltic Sea region, and a

non-migratory population has emerged in the wintering area in The Netherlands. This

thesis aims to understand these changes.

First, I studied the delay in commencement of spring migration. In the 1970s and 1980s,

barnacle geese commenced spring migration half April, whereas spring migration now

commences half May. I used a dynamic programming model to test three different

possible explanations of delay in migration: 1) Climate change, because geese follow a

green wave of fresh plant growth during spring migration, and are thus expected to be

sensitive to increasing spring temperatures. 2) Competition for food during stopover

because the population migrating to Russia has rapidly increased during the period in

which the migration change occurred. 3) Predation danger during stopover because the

number of avian predators such as White-tailed Eagles has drastically increased in the

Baltic stopover area. The model showed that a delay of one month is adaptive in both

the case of competition and predation danger. Strikingly, predation danger has received

very little attention so far in goose studies.

Migration strategy in geese is not genetically but culturally inherited, especially from

parents to offspring via an extended period of parental care. Because this thesis focused

on understanding migratory change, I focused on the parental care behaviour and the

parent-offspring association because a change in migration was expected to be preceded

by a change in the parent-offspring association. Because spring migration had delayed,

the question arose whether the termination of parental care also had delayed. This

would indicate a mechanistic link between the decision of commencement of migration

and the termination of care, and would allow the barnacle geese to continue transmission

of the migratory strategy to their offspring. Therefore, I quantified parental care

throughout the season from autumn migration in Estonia to wintering in the Netherlands

and through spring migration in Estonia. To quantify parental care, I compared

parental geese (geese with offspring) and non-parental geese (geese without offspring).

I showed that termination of parental care had not delayed but advanced as compared

to the earlier situation, leaving a gap of two months between the estimated end of parental

care (March) and the commencement of migration (May). This longer period of

‘adolescence’ and the accompanying exploratory behaviour may have strong influence

on the amount of new colonization attempts by these abandoned offspring.

In addition to delayed commencement of spring migration, also a non-migratory population

emerged in the Netherlands. Life-history theory predicts that 1) higher expected

Summary| 117

future reproductive success leads to shorter parental care and 2) decreased benefits of

parental care lead to shorter parental care. Both situations apply to the non-migratory

population as compared to the migratory population of barnacle geese. Migration is

a dangerous life-style, and has become even more dangerous as I showed earlier. Additionally,

the non-migratory offspring encounters few dangers, making the benefits of

parental care for the parents smaller. Hence, I compared the duration of parental care

between migratory and non-migratory barnacle geese. To this end, I also quantified the

parental care of the non-migratory population from autumn until spring. I showed that

non-migratory barnacle geese take care of their offspring 21% shorter than migratory

barnacle geese and terminate care already in November. This suggests a rapid adaptive

adjustment of parental care coincident with altered migration.

To understand the colonization history of the different populations of the Barnacle

Goose, I developed a set of 384 Single Nucleotide Polymorphisms (SNP) specifically

for the Barnacle Goose. By genotyping 418 individuals from Greenland, Spitsbergen,

Russia, Sweden and the Netherlands (all major populations) I identified significant population

structure. The results show that after previously having been separated, population

admixture occurs now between all populations, indicated by significant linkage

disequilibrium. Because the traditions of migratory behaviour promote differentiation

between populations, this admixture suggested that these traditions had broken or had

become weaker. We also show that the colonization of the Netherlands is not likely to

have occurred by the Swedish population (which emerged ten years before the Dutch

population emerged). The Russian and Dutch population are much more alike than the

Swedish and Dutch population, indicating colonization of the Netherlands by formerly

Russian barnacle geese.

In the synthesis I showed that we can use life history trade-offs as indicators of environmental

change. Based on the shortening of parental care I concluded that predation

danger is a more likely explanation for the commencement of spring migration than

food competition in the Baltic. I also showed that the shortening of parental care in

the Barnacle Goose is not the norm in the Anatidae family, where the form of parental

care is assumed to be very conservative. The observed change in our study showed that

either the non-migratory barnacle geese adjusted their parental care unprecedentedly, or

that the parental care systems in this family are poorly recorded or understood.

Finally, I showed with a mechanistic model of cultural transmission of migratory behaviour

that a delay in commencement of spring migration can explain sudden exploratory

behaviour and colonization of new breeding areas at the cost of increased offspring

mortality. The model also showed that the importance of culture on the transmission

of migratory behaviour strongly affected the rate of exploration of new migratory

strategies.