Anyone passing by on the roads that surround the rice paddies of the Tagus estuary does not even notice, but these days the water-covered plots are inhabited by thousands of sandpipers (slimy slime), preparing for the long journey to their breeding grounds in Northern Europe.
Attentive to their location and with several lost nights trying to temporarily capture some of these birds, placing small transmitters on them, biologist Afonso Rocha is part of a team of researchers from the University of Estremadura (Uex), in Spain, who want to understand how it is that changes and loss of habitat in the traditional stopping places of migratory birds are affecting their oxidative stress. It is that this can influence the future of these species. And, right now, the Tagus is the main stopover for sandpipers.
“Historically, the most important sites for the species as a stopping point, during the migration route, were Doñana, in southern Spain, and in Portugal the Tagus estuary, but due to drought and the increasing use of water by agriculture, in these areas of Doñana, and also in the Extremaduran rice paddies [espanhola]the available habitat has been reduced and the Tagus, in recent years, has also increased its rice paddy area and has been increasing its importance at a population level for the species”notes the biologist.
Flocks of 60,000 birds can arrive here, corresponding to 70% of the entire breeding population in the Netherlands. [agora designada Países Baixos]explains Afonso Rocha while taking a break in search of the various flocks of sandpipers that disperse across the rice paddies, binoculars pointed at a group resting in the distance, not far from a smaller flock of black ibis (Plegadis falcinellus).
The drop in the number of birds that use the neighboring country as a stopping point to regain strength, before heading to Northern Europe, is huge, says the Portuguese researcher. “In the rice paddies of Extremadura, although there are fluctuations, there could be between 5,000 and 20,000 birds, this year we counted just 1,500. In Doñana the numbers are even smaller, less than 1,000 birds, and there were around 50,000 there. It became one of the most important stopping places and many of those that were there now come to the Tagus.”
Between rice paddies in Extremadura and the Tagus estuary
Drought, the transformation of agricultural soils for fruit crops, such as peaches or almond trees, together with the abandonment of many rice paddies, causing flooded fields to disappear, are the main reasons for the loss of habitat for the Eurasian Sandpiper. -right in Spain. And this loss led the Uex team of researchers to decide not to capture the species only in the rice paddies of Extremadura, but to split up and also work in the Tagus, where Afonso Rocha is in charge of that function.
In addition, the estuary has another advantage – two “subspecies” of the right-billed sandpiper cross over there, the one that the biologist calls “nominal”, and which comes from Senegal, stopping in the Tagus to rest. It is from there that it then departs to reproduce in countries such as the Netherlands and Germany, traveling around five thousand kilometers along the entire route. The other subspecies, the “Islândica”, spends the winter here, and is also about to leave to reproduce in Iceland and, in smaller numbers, in the United Kingdom, covering a shorter distance, in the order of three thousand kilometers.
A fact that makes it possible to compare possible differences between the two. “If the birds that come from Senegal, for example, lose staging areas and have to travel longer, are they physiologically qualified or will there be consequences? Are they able to compensate for oxidative stress or not?” It is questions like these, asked by Afonso Rocha, that Uex researchers are trying to answer, with the investigation started a year ago and which will last at least until 2024.
Oxidative stress is measured by evaluating the balance between free radicals and antioxidants – if these components become unbalanced, this can alter the physiology of the species and, consequently, its conservation. That’s why, in the Tagus and in Spanish Extremadura, some birds are being captured to place transmitters on them and take a blood sample. The latter will make it possible to assess this stress and the transmitters will make it possible to follow the bird and help in the second phase of the investigation, which will take Afonso Rocha and his colleagues to the Netherlands.
It is there that, if all goes well, they will locate these birds that are now being marked, being able to test them again after the migration and also analyzing the blood of the offspring that may have, to see if “any information is passed on to the offspring ”, with regard to the possible compensation of oxidative stress.
Explained like this, it even seems easy, but the process is complex and is subject to many factors that can dictate its failure.
Tiny transmitters on the back
Of course, capturing birds is not easy. Afonso Rocha has 26 transmitters to install and has only managed to successfully install eight of the small devices on the back of female sandpipers (they are larger than males and, therefore, are chosen by biologists to carry the tiny devices that do not may exceed more than 3% of their weight).
Then there is always the possibility of something going wrong. – the day after having placed some transmitters, the biologist was worried about one of the birds, because after accepting the device and flying away, it began to transmit the signal, always from a fixed location, close by.
Afonso Rocha feared that something had happened to him, and he was right. – ended up finding only the remains of the bird and the transmitter (which could be reused). The suspect is that she hit one of the electricity cables and fell, and was later devoured by a bird of prey.
And, as if this were not enough, it is necessary that the devices have no flaws, that the birds are able to reach the breeding sites and that they are successful in the gestation and maintenance of offspring (which can be the target of predators and not survive). The difficulty is expressed in the numbers of the field work already started last year – at that time, 21 devices were placed, 13 birds that had them were later identified in the Netherlands, and of these it was only possible to recapture seven. Even so, Afonso Rocha says that the first year “went well”, and, with the increase in devices achieved this year, 2023 is expected to be even better.
For the time being, the wind has not allowed for a new night dedicated to capturing and placing transmitters on the birds, but the experience will have to be repeated, to try to place all the devices. Time is not so much anymore. From March onwards, the sandpipers begin to prepare to leave.
The rice fields in the Tagus estuary will be less full of life. At least until the end of the summer, when, if there are no significant changes, the big flocks will be back, passing through or staying there until the winter passes and it’s time to leave again.