Where do House Martins go for winter?
Summary
This trawl through the literature reveals that ringing returns are few and far between, that experiments with physical geo-locators ten years ago was abandoned, and that the most reliable evidence comes from analysing feathers for their isotopes that now roughly pin-point areas in sub-Saharan Africa where they visit for the winter. Recently a significant group of House Martins (HM) have been found in Kenya. It seems HM trade the temperate climes of North-west Europe in the summer for the tropical rainforests and savannahs of at least eight central and southern African countries for the rest of the year.
So where do they really go?
That is the biggest question about their natural history, yet it has defeated ecologists for a long time.
The British Trust for Ornithology (BTO) said. ‘Although House Martins migrate to Africa, the exact non-breeding range of House Martins that breed in the UK & Ireland is still not known.’[1]
You would have thought that it was impossible to lose track of 22,400,000-47,200,000 mature individuals.[2]
Analysis of BTO records indicates that in the British Isles about 1 million HMs have been lost since the 1990s.[3]
Of the ‘knowns’
i) we know that they go south; we see them gathering on the south coast waiting for the weather to break, and,
ii) we know that they tend to go mostly down the west coast of Africa.
Of the ‘unknowns’ it seems there is little evidence that they fly over the Sahara, but they might do, like a lot of other bird species, some smaller than House Martins.
After that there is little visual evidence as to where.
There are some maps that show a sprinkling of HM records down the Nile Valley (Egypt), but most of the activity is down the west coast of Africa. We are not here talking about any house martin subspecies from eastern areas including Asia that might use the Egypt route.
To put it all into perspective, there are three main areas in which HMs live. See the map shown on the on-line BIRDA map. [4]
i) The breeding area of Northern Europe including beyond the Arctic circle in Northern Finland, and across most of Russia.
It is important to note that part of this NW European area includes the coast of North Africa including Morocco and Tunisia. Although HMs breed along the Mediterranean coast of Africa, they also move elsewhere to winter, presumably southwards, i.e. from Africa elsewhere to Africa.
The North African HMs are amongst the same North-west HM group that also breed just across the water in Southern Spain (Extramadura). There three broods have been recorded.[5]
ii) The Sahara area which is a sub-optimal habitat that birds have to transit twice a year.
iii) Central Africa to South Africa which is where HMs disperse to see out the winter.
It is important to reflect exactly how these three areas that HMs are active in have evolved. As food (insects) becomes available in the northern part of the world with the sun getting higher in springtime, the birds migrate northwards, feeding and breeding. The Sahara gets in the way, but HMs have evolved ways of coping with this, by avoidance to the coastal west. As summer wanes in the north and food decreases the birds move south to the tropical forests and savannahs where insects remain available, and they rest there consolidating their populations until the next year. It is a journey that they have perfected since before man evolved.
Overwintering in South Spain – definite, but small numbers!
There is now evidence that some HMs overwinter in Southern Spain.
Liz Martinez who lives in Malaga, South Spain reports that, ‘several roost on my balcony from autumn to spring. Every year they arrive in my coastal town in October and stay until March’, complete with photographs, not masses of numbers. [6]
The North African regular breeders – but where do they go ?
There is a lot of research on HMs in Algeria where the Okbi hospital in Guelma City has 352 nests that have been researched extensively. They have two broods, arriving in February and leaving mid-October ‘leaving for ‘their usual winter quarters’.[7] So they have a short migratory pattern from presumably south of the Sahara to North Africa, but they do not know where the birds go in winter.
The Sahel
Flying over the Sahara is a monumental sub-optimal habitat that birds have evolved to master in a variety of ways (non-stop over, or around, east or west). With Climate Change occurring (presumably more rapidly than evolution can make adaptations), birds are becoming stressed to make this crossing.
The Sahel is regarded as a ‘Black Box’ inside which not a lot is known. The researchers [8] have reminded us that there is no such thing as normal rainfall in the Sahel, it is so variable, it provides the most dramatic example worldwide of climate variability
The Sahel goes through various major environmental periods; there was ‘The Sahel Drought’ of 1966-1984, and a period of rainfall during 1983-2019. Data from both these period showed that with the ‘aerial feeders’ such as HM the ‘average annual change in the population as a %’ actually increased by 0.7% very slightly during the drought period and fell by 2.1% during the humid period when more insects might be expected.
However, external factors brought on by humans with habitat loss are also significant impacts. There are different factors at work here, not all joined-up or understood. With the human population in sub-Saharan Africa increasing tenfold in the past century, and with 2% of the savannah being converted to farmland every year that will be a significant impact on HM. And we all know now that HM go to savannah during the winter.
Evidence is collected from a variety of methods (4)
1. Ringing
You would have thought that ringing HMs would bring in a lot of results with returns from Africa. But this is not true. Ringing returns are poor.
Tracking HMs from Belgium, Sweden and Hungary with small geo-locators has had to be abandoned for a variety of reasons in 2013 including low returns.[9]
3. Isotope indicators (endogenous markers)
The natural isotopes of any area can be identified from accumulations in feathers. So where birds have been living, the isotopes in their feathers gives a forensic clue as to where they have been.
Work on feather isotopes has been carried out by scientists in The Netherlands on returning birds. The feather isotopes fell into two ‘clusters’ which indicated different areas or ‘biomes’ of Africa.
i) “moist broadleaf forest in “cluster 1” and
ii) savannah mixed with tropical seasonal forest in “cluster 2”.
So we can at last get our heads around the sorts of habitats that our HM live in Africa, they are ‘tropical forests’ and ‘savannahs’.
The feather results also showed that birds from populations in Northern Italy went to Sierra Leone, Ivory Coast and Guinea, and another group went to Cameroon and Northern Congo.
As they said:
“Thus, while we strongly suspect that the Dutch house martins overwintered near Cameroon and the Congo, we cannot rule out the use of a band from Angola across Zambia … the far west forested regions of Sierra Leone, Guinea and Ivory Coast.’
Research on HM feathers by Cosme López Calderón at the University of Seville, Spain has indicated that the birds had been to four areas in West Africa on the Atlantic coast, eastwards towards Congo, as shown by the coloured map if you visit the website indicated.[10]
In another study scientists in The Netherlands using isotope data found that their HMs wintered in four distinct areas in Africa.[11]
The Southern African Bird Atlas Project (SABAP)
Combined data from many sources has revealed the presence of HMs in South Africa mostly in the south and South-east as shown in the following map.
The Southern African Atlas Project 2’ (SABAP2) uses a Citizen science recorded sightings that are logged into a database reporting rates and map distribution.
Thanks to Tania Anderson (Coordinator of the Southern African Bird Atlas Project (SABAP2). [12] and Jessica Wilmot (Flyway and Migrants Project Manager) for Birdlife South Africa for these maps.[13]
The above map shows lots of records around a city called Gaborone (Botswana), with more details shown in the following map.
Maps credit: © by kind permission from The SABAP2 team who would like to draw your attention to their homepage https://sabap2.birdmap.africa/ and cite the following article: [14]
4. Actual evidence of wintering en masse
One of our HM champions, wildlife cameraman, Benjamin Ward from Birds Beyond Borders Team [15 ] reports of a 10,000 roost site in Kenya (February 2024). He also reports of a bird group called A Rocha which rings a handful of HMs every year in the Rift Valley. Look out for his new film.
Conclusions
So the sub-Saharan countries in which HM over-winter include the following: Botswana, Cameroon, Congo (Northern), Guinea, Ivory Coast, Kenya, South Africa, Sierra Leone, as well as Madagascar (vagrant only).
Written by Dr John Feltwell
Trustee
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[1] House Martins https://www.bto.org/understanding-birds/birdfacts/house-martin (accessed 20 Feb 2024).
[2] BirdLife International 2015. https://datazone.birdlife.org/species/factsheet/northern-house-martin-delichon-urbicum/text (accessed 02 April 2024).
[3] Paul Stevens, 2024. Analysis. 2 April 2024 on What’s Ap. House Martin Chatter.
[4] BIRDA, Common House Martin. https://app.birda.org/species-guide/24867/Common_House_Martin (accessed 20 Feb 2024).
[5] Pajuelo, L., F. De Lope, and E. Da Silva. 1992. Breeding Biology of the House Martin (Delichon urbica) in Badajoz, W Spain. Ardeola 39: 15–24 https://www.ardeola.org/en/volumes/391/articles/15-23/ (accessed 20 Feb 2024)
[6] @LizMartinez, Facebook, 12 Nov 2020.
[7] A. Khaled, H. Fenghour, M. Bara, S. Atoussi, M. Rouaiguia, A. Reggam, I. Houhamdi, M-L. Ouaki, and M. Houhamdi, 2021. On the breeding phenology of the common housemartin Delichon urbicum in Guelma city (northeast of Algeria). Bulletin de la Société zoologique de France,146(3):123-127 https://www.researchgate.net/publication/355198134_On_the_breeding_phenology_of_the_common_housemartin_Delichon_urbicumin_Guelma_city_northeast_of_Algeria (accessed 20 Feb 2024)
[8] Zwarts, L, Bijlsma, R.G. and Kamp, J van de, 2023. The Fortunes of Migratory Birds from Eurasia: Being on a Tightrope in the Sahel. https://bioone.org/journals/ardea/volume-111/issue-1/arde.2022.a29/The-Fortunes-of-Migratory-Birds-from-Eurasia–Being-on/10.5253/arde.2022.a29.full (accessed 20 Feb 2024).
[9] Tracking House Martins. https://www.bto.org/our-science/projects/house-martin-survey/tracking-house-martins (accessed 20 Feb 2024)
[10] C.López Calderón, 2017.The winter ecology of House Martins. https://bou.org.uk/blog-lopez-calderon-house-martins/. This is published by The British Ornithologists’ Union (BOU). This reference is linked with the following reference (accessed 20 Feb 2024). C.López-Calderón, K. A. Hobson, A. Marzal, J. Balbontín, M. Reviriego, S. Magallanes, L. García-Longoria, F. de Lope, Anders P. Møller. 2016. Wintering areas predict age-related breeding phenology in a migratory passerine bird. https://doi.org/10.1111/jav.01070 (accessed 20 Feb 2024)
[11] Hobson, K.A., Wilgenburg, S.L.V., Piersma, T., Wassenaar, L., (2021) Solving a Migration Riddle Using Isoscapes: House Martins from a Dutch Village Winter over West Africa. PLOS. https://pubmed.ncbi.nlm.nih.gov/23028734/ (accessed 20 Feb 2024).
[12] The Southern African Atlas Project 2’ http://sabap2.birdmap.africa/ (accessed 20 Feb 2024).
[13] https://sabap2.birdmap.africa/ (accessed 19 March 2024).
[14] Brooks, M., Rose, S., Altwegg, R., Lee, A. T., Nel, H., Ottosson, U.Thomson, R. L. (2022). The African Bird Atlas Project: a description of the project and BirdMap data-collection protocol. Ostrich, 93(4), 223–232. https://doi.org/10.2989/00306525.2022.2125097
[15] https://birdsbeyondborders.com/ (accessed 2 April 2024)
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