Let's talk biology first. Thrushes belong to the muscicapidae family, a sub-family of the turdidae. The genus turdus comprises 67 species of birds. Of these, only 4 concern us. We will introduce them to you below. Do you know why game birds migrate and what influences their migration?
Among sedentary birds, some species occupy a particular territory. They never leave it, regardless of the time of year. Others occupy a territory only during the breeding season. Migratory birds move to other regions during the wintering period.
Since the last ice age, birds have been forced to migrate in search of milder areas, both in terms of climate and food. This is why the Mediterranean area has been a suitable breeding ground.
It is easy to imagine that this problem is related to the lowering of temperatures. As insects become scarcer, birds are forced to look for food elsewhere or even alter the proportions of certain elements of their diet.
Game birds thus adapted by feeding mainly on insects or invertebrates in the summer and opting for berries and fruit in the winter.
Global warming combined with the pressure difference between the Azores anticyclone and the Icelandic low pressure system also play a role in bird migration. Over the last thirty years, this phenomenon has resulted in more frequent westerly winds, leading to rainier and milder winters. As a result, small game populations are being forced to delay or even discontinue their migration.
By way of example, 30 years ago the migration probability index of the wolf thrush in France was 7/10, i.e. seven years out of ten, today it is 2/10, meaning they only migrate to France 2 years out of 10.
Wind also plays a role in migration. When undertaking a long journey, birds differ in the way they use the wind. Game birds decide on the flight altitude for which the wind is most favourable. Radar studies confirm that they migrate with greater intensity in the presence of a tailwind and in headwinds or sidewinds to limit the loss of energy and time.
The creation of intense luminescent sources such as lighthouses and industrial sites, etc., can be an indirect aid to tracking during the migratory journey. However, they can also be a deadly hazard for birds.
Changes in the agricultural landscape also affect migration, especially if they are linked to the loss of feeding sites. The problem lies in the fact that even though birds have a great capacity to adapt, they no longer have sufficient time to adapt so quickly, resulting in significant changes in migratory movements.
Studies enable us to determine the influence of climatic variations on bird migration. Europe's climate is largely dependent on an index called the North Atlantic Oscillation (NAO). The NAO Index provides a measure of the difference between the Azores anticyclone and the Icelandic depression.
It has been observed that if the Icelandic depression is strong and the Azores anticyclone is weak, we generally have cold and dry winters in Europe and therefore a chance of heavy migrations. Conversely, if the Icelandic depression is weak and the Azores anticyclone strong, we have mild and wet winters, and therefore low migrations.
Since 2008, the NAO tends to shift from positive to negative, which is a good sign for future migrations.
- MIGRATION: The music thrush is the most well-travelled of all. It is the first to undertake its post-nuptial migration. It can be spotted as early as mid-September. It is also the latest to return to the nesting grounds.
It does not move in large groups, travelling mostly at night on clear days. Its movement is essentially motivated by the search for food. It is not uncommon to find a large number of birds in a hedge loaded with berries.
When they are camped in an area, music thrushes wake up at dawn. They travel for approximately one hour in search of feeding areas. In the evening they return to their resting places in the woods or large coppices shortly before sunset.
- BEHAVIOUR: The music thrush is very cautious and therefore quite suspicious, always lying low. Although they are relatively easy to call, at the slightest suspicious noise or false note, they will fly off.
It has a more sedentary or even semi-migratory behaviour. Only the northern and eastern populations are considered migratory.
- MIGRATION: The drake thrush is not a highly migratory bird. Post-breeding migration takes place in France in early October continuing until November. Pre-breeding migration takes place from late February to April. It does not travel at night. They fly in flocks or small groups.
- BEHAVIOUR: This is the most feral of the thrushes. It is constantly surveying its environment. It spends most of its time in trees, only coming down to the ground to drink or look for insects.
- DISTRIBUTION: It is mainly migratory. Only a few populations are partially migratory. Populations in northern Russia and the Baltic States travel southwest. They are found in England, Portugal, France, and southern Italy.
- MIGRATION: Birds begin their migration in mid-October, reaching southwestern Europe in November. However, if the cold weather appears earlier in northern Europe, They may start their migration as early as the beginning of October. Pre-breeding migration takes place from late February to April with a peak in March. The wolf thrush travels at night as well as during the day.
- BEHAVIOUR: The wolf thrush has a gregarious instinct, often living in colonies. If a flock lands, one bird is enough to make the whole flock join in. Its flight is brisk and fast. It is a wary bird that is rather difficult to approach. It does not respond easily to calls, especially when in a group.
- DISTRIBUTION: This species occupies broadly the same areas as the wolf thrush. However, the species is more widespread in Central Europe, as far as the Alps and the Massif Central.
- MIGRATION: the litorne thrush is very sociable, living in colonies within the species but also with the wolf thrush. They travel and search for food in groups. The Central and Eastern European populations only migrate during periods of severe cold and food deprivation.
Pre-breeding migration extends from the end of February to the end of March. Pre-breeding migration takes place from late February to late March.
- BEHAVIOUR: litorne thrushes travel in large flocks, often during the day like the wolf thrush. The main motivation for travel is food. This is the final thrush to arrive in our country in autumn. The colder the weather, the more abundant the litorne. If the weather gets milder, they tend to move to higher ground.
They often stick to the ground to feed. They respond well to the call, coming even if one imitates the song of the wolf thrush.
"The thrush odyssey", Jean-Paul FLORENTINO"