ICTS Doñana | The Doñana passerine bird ringing campaign detects that trans-Saharan migrants arrive with more body fat
European pied flycatcher (Ficedula hypoleuca) in Doñana / Credit: José Luis Arroyo, EBD-CSIC
The ringing campaign conducted by the Bird Populations Team within the Environmental Monitoring Area of the ICTS-Doñana (Doñana Singular Scientific-Technical Infrastructure), Doñana Biological Station - CSIC, has detected that passerine birds migrating to tropical Africa through the Sahara Desert arrive at the National Park with more body fat, which could mean that they need to stop for less time to accumulate fat.
This is a change that is being detected after 31 years of ringing campaigns and has been become more evident in the last three or four years. José Luis Arroyo, from the Bird Population Team and coordinator of the campaign, explains that there is a tendency in the arrival of the birds: ‘Body fat is measured visually, observing certain areas of the bird based on predetermined codes: from 0 (no fat at all) to 8 (body completely covered with fat). In the first year of the campaign in 1994, the average fat was only a little more than 2, but the average for the last four years is one and a half points higher.’
‘All migrants need to accumulate body fat. It is the ‘fuel’ they use to fly long distances. This accumulation is a costly process and only done if they are going to migrate. There are some geographical areas, called stopover areas, where these species stop specially to accumulate fat, as they have good conditions for obtaining food in a short space of time. Traditionally, Doñana has been one of these stopover areas because it fulfils these optimal conditions, as well as being very close to the next barrier the birds encounter, the Atlantic Ocean, before continuing their migration southwards,’ Arroyo points out. Trans-Saharan migrants are the first migrants to pass through, between summer and early autumn, as opposed to short-distance migrants, which spend the winter in the Mediterranean basin, in our case, the Iberian Peninsula and North Africa. The most common species captured by the ringing campaign are the European pied Flycatcher (Ficedula hypoleuca), the garden warbler (Sylvia borin) and the willow warbler (Phylloscopus trochilus).
According to the ICTS-Doñana technician, this fact may affect the passage of these birds through Doñana in several ways: ‘If this was previously a more or less obligatory stopping place to accumulate fat, now some of these long-distance migrants may not need to stop here for so long because they have already accumulated this resource in areas further north. This also means that they are less likely to be caught. According to historical data, long-distance migrants or trans-Saharan migrants are being caught in decreasing numbers in the ringing campaign, and this is normally associated with a decline in the populations of the most common species at European level. But, in addition to this population decline, a greater amount of body fat could mean that a significant number of individuals fly over Doñana almost without stopping and are hardly detected, as many of them are nocturnal migrants and can fly for several days at a time. ‘Some, the fewest, come from quite distant areas, such as the European part of Russia, to winter in West Africa, although most come from northern and central Europe and the British Isles,’ he notes.
The relationship between temperature and food
Temperature is directly related to the food available. ‘Migratory passerines cope well with relatively low temperatures, if food is available, but this is often not the case. Most of them are insectivorous birds and food availability goes down if conditions are not good, cold or rainy, when insects are not as available to them. Traditionally, in areas in the north of the Iberian Peninsula or in central and southern France, the weather conditions during the migration of these birds were less favourable than they are today, but in this geographical area the good weather conditions in summer tend to be longer’, says José Luis Arroyo.
Abdominal fat in garden warbler (Silvia borin) / Credit: José Luis Arroyo, EBD-CSIC
This could explain part of the drop in captures of these species since the beginning of the campaign. As they do not need to eat so much during the day, many of the individuals spend their stay resting, thus reducing their mobility around the area and the possibility of being captured in the ringing campaign.
In short, the number of trans-Saharan bird captures has decreased. In this sense, Arroyo explains that ‘they are the ones that adapt the worst to the changes because they go further away and are not able to adapt their vital calendars to the possible changes that could be occurring in the breeding areas, thousands of kilometres away. However, those that stay closer to their breeding grounds during the winter can anticipate their return more easily and are more adaptable to the changes they would encounter in their breeding grounds.
In pre-Saharan or short-distance migrants, the trend of common chiffchaff (Phylloscopus collybita) over the historical series remains stable, although the numbers of Eurasian blackcap (Sylvia atricapilla) and European robin (Erithacus rubecula) are decreasing. In these two species, which are maintaining or increasing their populations at European level, it could be that some of the birds are staying closer to the breeding areas during the winter, due to the climate in more northerly latitudes.
The importance of ringing campaign data
These small birds are good indicators of global change, as their short lifespan means that variations in their life cycles can be seen in just a few generations. As they live an average of three to five years, there are new generations every few years.
José Luis Arroyo says that the scientific ringing campaign is the tool for finding out what is happening to this group of birds: ‘Doñana is very important for many species, including passerines, which can pass through several million individuals during the migration season. We therefore have a responsibility to know what is happening to them and scientific ringing campaigns are one of the best monitoring methods to detect possible long-term trends or changes. We are the alarm bells that go off when something changes.
‘If we talk about biodiversity, the number of species is higher than other groups that we think are very important. For example, nobody doubts that the raptors of Doñana need to be monitored, but passerines, among which there are also some species that have an unfavourable conservation trend, are not usually given as much attention. However, some of them are common birds that we have in our gardens and squares,’ he says.
Almost 92,000 catches in 30 years
The Manecorro constant-effort ringing station was first installed in 1994 on the southern shore of the El Rocío marsh, in the northwest corner of the National Park. Since then, it has been active every season during the post-nuptial passage of passerines and associated groups, with a total of 91,960 captures, of which 1,528 birds belonging to 53 species were made in 2023.
Although the number of ringing days may vary from one year to another, for example, due to weather conditions, the campaign is always carried out with a similar methodology so that the data are comparable over time. Specifically, in the 2023 campaign there was a slight increase in the average number of daily catches compared to the previous season, from 29.4 to 31.2.
José Luis Arroyo / Credit: Rubén García Olivares, EBD-CSIC
José Luis Arroyo has participated in all the passerine ringing campaigns in Doñana National Park. In the first one as a volunteer ringer and later as a worker at the Biological Station, he has been linked to the Park for most of his working life, where he started as a guide and environmental educator. Now, he carries out his work together with Rubén Rodríguez Olivares and Iván de la Hera, also ringers, and the rest of the members of the Bird Population Team: Antonio Martínez, José Luis del Valle and Giulia Bastianelli.
A large number of volunteers also participate in this campaign every year (more than a thousand in these 30 years of campaign) who help in the most common tasks of the station and receive training that helps them to obtain the certificate of aptitude for the scientific ringing of birds
About ICTS Doñana
The ICTS-Doñana, an infrastructure dependent on the Doñana Biological Station - CSIC, is the largest field laboratory in Europe and a research tool for the scientific community. It supports research in Doñana and serves to generate decisive information for good management and conservation of the protected area. It is a unique place to investigate the ecology, evolution and conservation of biological diversity and the impact of global change.
This infrastructure provides accommodation, laboratories, equipment, office space and scientific, technical and logistical support to researchers. Access projects are approved by the ICTS Doñana Access Committee.
The Singular Scientific and Technical Infrastructures are unique and exceptional facilities of their kind. They carry out cutting-edge research of the highest quality and act as centres for the transmission, exchange and preservation of knowledge, technology transfer and the promotion of innovation. They are publicly owned infrastructures recognised by the Ministry of Science, Innovation and Universities, unique and open to competitive access. There are currently 29 ICTS in Spain and every four years they are evaluated by an advisory committee.