Age-related differences in songbird migration performance in autumn

Abstract

Migration to wintering grounds is a process that a large proportion of songbirds perform every year. It has been suggested that there are differences in the migration performance between adults and juveniles due to the experience the former have over the latter, after having flown this journey before. The aim of this study was to analyse age differences regarding activity, energy deposits and departure orientation during autumn migration, to provide better insights on how adults and juveniles may vary in behaviour and ecophysiological adaptations. I captured 132 songbirds from 11 species (both first-year migrants and adults) in southern Sweden and tested them in outdoors (Emlen-funnels) and indoors (circular cages) experiments to estimate their activity and preferred orientation. Juveniles were more active than adults, most likely due to the exploratory flights they perform prior to migration in order to familiarise with the area and to develop their stellar compass. I found no effect of age neither on size of energy deposits nor on departure orientation, meaning that both age groups carried similar sizes of fuel load and perform equally well when choosing their preferred migratory direction. One of the study species was the vagrant yellow-browed warbler Phylloscopus inornatus, which showed a south-east direction, instead of an expected south-west orientation as predicted for the location of the study. Several birds from different species showed preferred reverse directions, and although it has been proven that this behaviour may depend on the energetic status (leaner individuals perform reverse migration), the results did not support this idea. One possibility is that reverse orientation may be related to immune function responses.

Popular Abstract

On the road again or rookie driver?
A study of migration activity and orientation in adult and juvenile songbirds

During summer and autumn, birds all around the world leave their homes to spend the winter in warmer places, which are usually far away. Just imagine a European robin you see around your area, maybe in Copenhagen, which flies all the way to Morocco to winter there. This is what we call autumn migration. For juveniles that have been born only a few months before, it is the first time they fly this journey, which may seem a big challenge if we keep in mind that they need to do it on their own. Fortunately, they can use the stars, the sun and the Earth’s magnetic field to find their way as a compass. Older birds have travelled this same journey on previous years, so they probably also remember landmarks along the way and know better than younger birds how to successfully arrive to their destination. We can therefore think: are there any differences in timing, navigation cues used and migration performance between adults and juveniles, considering adults have more experience than juveniles?
I captured 132 birds of 11 species, both adults and juveniles, in southern Sweden, to determine if there was any variation between the age groups regarding their energy deposits (how fat birds are), orientation (direction birds want to follow) and activity during autumn migration. I wanted to study if adults were fatter, because they may be better at finding food than juveniles, that still need to learn. For this, I measured how much fat the birds had accumulated, which is shown as yellowish patches on the belly. I also expected adults to be more certain of their migratory direction, because they had migrated at least once before, unlike juveniles. How can we test what orientation a songbird wants to fly to? After sunset, when birds usually start migration, I placed each bird for one hour in a funnel-shaped cage, called Emlen-funnel, with a thermal paper attached to the inner slopping wall. When the bird started moving inside the cage, the claws made marks in the soft pigment of the paper as if they had drawn them with a pen. I could then look where the marks were more concentrated, and estimate the direction the bird wanted to fly in while being in the cage. Afterwards, I placed the birds in another circular cage where I could record the activity with a camera for one day. These videos and the number of marks in the paper would tell me how active the birds were.
Surprisingly, adults and juveniles were equally fat. Both groups were able to choose their direction correctly, but juveniles were more active than adults. Just before migration, birds are very active, flying around and looking for food that will be stored as fat. The fat will be used as fuel for their flight. Both adults and juveniles need to find enough food, so this would explain why there are no differences in fat, but not why young birds are more active. It seems that this high pre-migratory activity would help juveniles to learn how to use the stars, the sun and the Earth’s magnetic field to find their way, and to familiarise with the area (because they will need to come back to it in the spring). But these two processes adults do not need to go through again, and may instead remain calm and not spend energy. If birds are not able to develop this orientation sense to be able to return to important locations, they will get lost and arrive to wrong places, like what may happen to so-called vagrant species. I was lucky enough to find one of them on passage at my study site in south Sweden, the yellow-browed warbler (Phylloscopus inornatus), which normally would spend the winter in south-east Asia. This vagrant behaviour remains a mystery, especially considering that in recent years they have become more common in southwest Europe. Maybe some of them want a change and prefer to spend their Christmas in a new place?

Master’s Degree Project in Biology, 45 credits, 2020
Department of Biology, Lund University

Advisor: Susanne Åkesson
Evolutionary Ecology Department

• author: Martínez Hernández, Elena
• supervisor: Susanne Åkesson
• organization: Degree Projects in Biology
• course: BION02 20192
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9022096
• date added to LUP: 2020-06-24 14:27:41
• date last changed: 2020-06-24 14:27:41