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The Science of Migration

The science of migration

The Atlas of the birds of the Eurasian and African areas has been published. It is an extraordinary work and fundamental for knowledge of the phenomenon of migration, for protection of huntable species and to analyse the effect of climate change on species

Marco Gustin, Head of Species and Research

Black Kite

The long awaited work has finally come out, one of the most challenging works on bird migration at a planetary level. We are talking about The Atlas of the Migration of Euroasiatic-African Birds, introduced at the end of spring at Ventotene. It was carried out on behalf of the Ministry of Ecological Transition and the Italian Government by some of the leading world experts in the field of bird migration, among them Franz Barlein of the Institute of Avian Research at Wilhelmshaven, Fernando Spina of ISPRA and Roberto Ambrosini of Milan University. It covers an enormous geographical area, represented by two continents, Europe and Africa, along the so-called palaearctic-african flyway. Using data gathered from the process of ringing in Europe for more than a century and held in the Euring data bank (the centre in charge of monitoring, co-ordination and management of ringing activity within the scientific community) the movements in time and space of 300 species of bird were analysed and mapped.

In some cases (limited to 100 species) the Atlas integrates the data on the movements of individual species, obtained using mathematical models of migration, by tracing the movements of ringed individuals. In other cases the routes of subjects were found using electronic devices, principally satellite transmitters, these principally GPS-GSM or geolocalisers, adding further and more complete information about the routes of migration between Europe and Africa of single species.

The work comprises 4 separate research modules which deal with different aspects of the migration of birds. The first relates to the historic changes in migration patterns, the second the intentional culling of birds by man, while the third deals with the phenology of migration of species that are the object of hunting activity. Lastly the fourth analyses the migratory connectivity within the palaearctic-african flyway. We will look at them one by one.

How migration is changing

For the conservation and management of bird populations it is of fundamental importance to study how the movement of birds changes along the palaearctic-african flyway. Variations could influence the seasonal distribution of populations leading to a rise or a fall in their numbers. For example global warming could induce a greater number of migrants to winter further north or to move shorter distances. And it is with precisely this in mind that the Atlas examines the distribution of the re-capture of 128 species. With what results? In general the majority of the species has not demonstrated a significant variation, but some changes are however highlighted. Most of the birds reduced the migration route by 3.7km per year on average, both wintering further north than in the past and becoming more sedentary. And for nearly a third of species individuals who have become sedentary have increased. The white stork, for example, has continuously reduced its migration radius over time and the winter distribution of individuals has moved from southern and eastern Africa to the Iberian peninsula and west Africa. Even for waterbirds the Atlas reveals a reduction in the average migration distance.

Culling of birds

And that brings us to the second module of the Atlas, which concerns the deliberate killing by man. As established by the Birds Directive, the deliberate sampling of birds of the wild population can be carried out only on condition that it is sustainable. In demographic terms the loss of individuals as a result of deliberate killing or capture should not add to those caused by natural factors (e.g. by predation or illness) that impinge on the numbers in the population. It has to be said that the demographic effect of the capture or killing by man of individuals on a population varies in relation to the number, the gender and the age group of those taken, as well as the phase of the annual cycle in which it happens. And it is precisely for this reason that the international legislation takes into account these aspects, by providing measures to promote sustainable sampling.

In this context the Atlas assesses the causes of changes and trends in mortality derived from the killing of birds by man based on geographical, historical and seasonal analyses as well as with regard to different methods of killing and different taxonomical groups. This is because deliberate killing, especially if illegal, is a problem relevant to conservation. And because, despite the majority of European countries having aligned their own legislation to international treaties, illegal activity continues to threaten the populations of European birds.

Italy, poaching on the increase

Concentrating on species listed the Birds Directive Attachment I (species strictly protected) and Attachment II A (huntable species in the whole EU) the authors have highlighted an historical trend of a decrease in reports of poaching against those species which are strictly protected and are also the most endangered. Analysing the geographical distribution of black spots and considering the protected species a substantial improvement in the Mediterranean region (including the Balkans and Tunisia) is noted. The problem in the black spot of the Middle East remains unchanged. In Europe the situation generally improved after the Birds Directive came into force, even if in Italy, locally, this problem is unchanged in sites both in the north and south of the country. In Africa the location of the black spots has moved over time from the north western countries to those of the south east. Analysing the hunting season instead the frequency of reports of illegal killing has only changed positively in the U.K. and Norway. In other areas of black spots identified before the Birds Directive there has been a marked deterioration in countries such as Portugal, Italy, Greece, Albania, Finland and the area of the Pyrenees.

Hunting and pre-nuptial migration

After deliberate killing the third module of the Atlas deals with the phenology of the species that are the object of hunting activity. The key concept of article 7, paragraph 4 of the Birds Directive is to supply information on the breeding times and pre-nuptial migration of the species listed in Attachment II. These species can be hunted according to national legislation but not during the breeding season nor, for migratory species, during migration to the breeding areas. So knowledge of the times of pre-nuptial migration is essential to ascertain the appropriate period to hunt.

To analyse these periods which are so important for the life of birds the authors of the Atlas used data in the EU ringing bank (Euring) on ringed birds and their recapture. The analysis used two different methodological approaches: the first using analysis of movements in space and time of each singular ringed individual that was subsequently recovered, alive or dead. The second used modelling of the process of space and time migration to the reproductive areas, taking into account every encounter of a ringed and/or recovered bird in a certain geographical area.

The confirmation of the Italian “Key Concept”

To assess the timing of pre-nuptial migration only individuals which moved north for at least 100km in a direction between 135-315 degrees were analysed. This is because, in Europe, return migration movement is, for the most part, directly towards the north, northeast or northwest. Of the 80 species listed in Attachment II of the Birds Directive as huntable in the member states only 57 species supplied sufficient data for recovery analysis and/or mapping from the start of pre-nuptial migration. But that which results from the researchers’ analysis also has, and will have, a major significance for Italy because it confirms the validity of the so-called Italian “Key concept” with respect to the dates of the start of pre-nuptial migration. A substantial proportion of migratory species already leave between the 1st and 2nd 10-day periods of January as is the case for thrushes and waterfowl (and Common Teal in particular). It follows that hunting, in these periods, must be prohibited in our country and, in general, in Mediterranean countries (for example France): the practice if these periods are exceeded constitutes a serious violation of the Birds Directive.

Connecting to the world

It is defined as the link between the periods and the areas in which the migrants spend the different phases of their annual cycle. We are talking about migratory connectivity, which in the scope of the Atlas, as the 4th module of analysis, 137 species of bird were studied by analysis of recapture. Overall the migratory connectivity of birds within the palaearctic-african flyway system has shown a high degree of variability from species to species but nearly all those analysed (around 92%) showed a significant connectivity. This means that the individuals are well connected to others while they move in time and space facilitating good environmental policies to be shared and better linked across different countries. In every case the migratory strategies vary even within single species: the majority have demonstrated migratory populations that are geographically distinct. This means that populations of the same species are found in different areas. The Atlas has compared the migratory connectivity between sedentary species, those partially migratory or trans-Saharan  and between passerines and non-passerines also taking into account the body mass of the species.

Long migration, less connection

Resident species demonstrate the highest level of migratory connectivity, partial migrants an intermediate level and the sub-Saharan ones the lowest values. Furthermore the largest species tend to show the highest level of migratory connectivity, while no significant difference emerged between passerines and non-passerines. On this point previous studies had shown that migratory connectivity diminishes with the distance of migration. Indeed the sub-Saharan migratory species, which cover the greatest distances have the weakest connectivity. In addition the largest species live the longest which favours social transmission of the migratory routes to non-breeding areas and helps to preserve the migratory knowledge across the generations: i.e. it could have favoured a greater population mixing in the smaller species.

The analysis carried out in the Atlas was able to identify with precision the models of migratory connectivity specific to many species thus supplying a fundamental instrument to improve conservation strategies for, and management of, target species.

By quantifying and describing the strength and models of migratory connectivity among European bird species it is hoped that the analysis arising from this work can improve the understanding of the migration of birds, and, from a practical point of view, facilitate the conservation and management of birds at a population level. And then the effort in carrying out such a complex and important task will not have been in vain.

The Eurasion African Bird Migration Atlas. Euring/Cms edited by Spina F., Baillie, S.R., Bairlein, F., Fiedler, W. and Thorup, K., Eds, 2022

To consult the Atlas:


300 species analysed by the Atlas

8 geographical areas of Europe

137 species studied for migratory connectivity

3.7 km a year travelled less by migratory birds.

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