Birds have developed specific and complex behaviors! It’s easy to get lost in them! And that’s a good thing!
Humans have a tendency to want to organize, categorize, and classify everything. This can be contrary to the very concept of biodiversity. Biodiversity is the result of a multitude of connections between a living species and its ecosystem. Various factors come into play: physical, chemical, climatic, etc. But also biological ones (seasonality of food resources, refuges, interspecific links, or links between individuals of the same species, etc.).
Conversely, it is important to identify trends and major concepts to better understand our environment and thus better preserve it! In this article, we aim to discuss the concept of migration. And, as promised in the previous article, to connect it with the concept of biodiversity and the initiatives generated by PlantC.
Following the signals…
Migration is a periodic, long-distance movement of an animal species. Many birds, fish, and other animals perceive environmental signals that allow them to identify when to undertake migration.
This behavior (like reproduction) therefore follows biological rhythms with a cycle longer than the circadian rhythm, specifically an annual cycle. These are known as circannual rhythms. They are primarily governed by food availability and the light/dark period. Finally, genetic differences can also lead to variations in migratory behaviors within the same species.
Scientific research demonstrates an ability to orient themselves using the sun. Nocturnal animals are believed to navigate by the position of the North Star, which remains constant throughout the night. Finally, the Earth’s magnetic field is also thought to play a role.
But let’s return to our birds… Migration is therefore defined as a periodic, usually annual, movement between a resting area (or wintering ground) and a breeding area, including the return journey (Figure 1).
But why?
In winter, the temperature drops. However, birds are homeothermic animals, meaning their body temperature is constant (approximately 41°C). To maintain such a temperature, they need “fuel,” i.e., food. In winter, food resources are less abundant (e.g., fewer flying insects) and their accessibility is more complicated (frost, snow, etc.). Therefore, the migrant must also anticipate this scarcity and build up its fat reserves before departing (both for the outward and return journeys).
Birds have developed different behaviors. Some species are sedentary. This means they will occupy the same area throughout the year and year after year. Examples include the House Sparrow, the Common Moorhen, owls, the Eurasian Nuthatch, woodpeckers, the Magpie, the Grey Wagtail, etc. Sedentary species can have an adapted, more diversified diet (granivorous during lean periods, and insectivorous in high season (larvae, caterpillars, etc.). This is notably the well-known case of Tits, the Corn Bunting, the Grey Partridge, etc.
Others are called migratory. They are referred to as partial or complete migrants. A complete migrant is generally a strict insectivore, such as Swallows, the Common Swift, or the White-fronted Redstart. However, there are bird species for which migration is not very clear. These are called partial migrants. Nevertheless, it depends on the country where you are observing! Indeed, for the same bird species, there can be groups of individuals (populations) showing different migratory behaviors. For example, populations from Northern Europe will be considered total migration populations by a Norwegian. But Mediterranean populations will be sedentary species for local observers!
Let’s take the European Robin. You might believe that “your Robin,” present in your garden in Belgium since summer, never leaves you in winter!? In fact, it is highly probable that the individual you observe in winter is an individual that comes from Northern Europe. “Yours,” which kept you company this summer, has already left for the South!
The Ecological Network and Migration
The diversification of habitats is of utmost importance! Yes to the planting of hedges and trees, but not exclusively! It is essential to develop wooded areas, forest edges (true ecotones!) but also to preserve open areas (agricultural plains) with or without buildings, fallow areas (grassy strips, field margins) or an unmown garden!
The bocage areas are generally described as providing “shelter and food” (Figure 2). Sometimes useful as habitat for certain species, sometimes useful as food resources for others. Did you know that the berries of trees/shrubs are often red? Birds see very well in reds. Furthermore, the presence of bocage areas is particularly important for species with creeping migration. That is, for individuals who move from one “bush” to another. Unlike other migratory species that will fly very high during this period.
But habitat diversification is essential! The “imperfections” of houses are important for providing refuge areas for birds. E.g.: sparrow in winter. Finally, more grassy areas will be favored by species that enjoy staying on the ground, or having an unobstructed view without trees.
Also good to know, bird species are mostly diurnal but migrate at night, safe from predators (and they will eat during the day).
Here are a few examples
Low hedges are appreciated by Grey Partridges, which are sedentary but whose populations are in sharp decline in our country. Skylarks avoid bocage areas and proximity to woods, or vertical elements, which are real perches for predators! Moreover, they hardly appreciate limited visibility. The same applies to the Western Yellow Wagtail. The Red-backed Shrike appreciates open, discontinuous, mostly thorny hedges close to late mowing.
The Corn Bunting is a partial to sedentary migratory species. A gregarious species in its wintering territory, near hedges or trees, but during the breeding season, individuals will avoid these environments, preferring open areas. The Yellowhammer, a partial migrant, also appreciates open environments delimited by hedges/bushes. The European Greenfinch will feed in an open environment but will use hedges for nesting. Hawthorns and Elderberries will be important for it during the bad seasons (partial migration). The Common Linnet and the Common Whitethroat will also appreciate hedges for their nesting.
In conclusion
As we announced from the introduction of the article: long live the complexity of ethologies and environments! Are you interested in the phenomenon of bird migration? Visit the BeBirds website! Migration will hold no more secrets for you.
In our next articles, we will endeavor to illustrate these points with concrete examples.
Sources :
- Reece, Urry and al., Biology, Campbell. 9th Ed. 1458 p.
- Svensson, Mullarnay & Zetterström. 2015. The Bird Guide. Delachaux Edition, 446p.
- Michaël Leyman, Olivier Dugaillez, Damien Hubaut & Stéphane Claerebout. 2021. Introduction to Ornithology. Nature Guide Training. Ed. Circles of Naturalists of Belgium asbl.
- D. Hubaut. CNB-LDN Outing 78 – Migration of Countryside and Bocage Birds in Rebecq, October 6, 2022.
- Bourdouxhe L., SPW-DGO3, Directorate of Hunting and Fishing. 2016. Wildlife of Agricultural Plains: Understanding it Better to Preserve it . 2nd Ed.
- Hubeaux Damien. 2020. Mahaie.be. Association for Agroforestry in Wallonia and Brussels.
