The gathering

The walk home has a certain poignancy this evening. Above the riverside limes are swallows and house martins, a gathering of individuals feeding up before departing south. The chattering calls imply a sense of nervous anticipation and emphasise that these evenings will soon slip towards a silence that is broken only by the solitary robin, now delivering his winter song. Only a few brief weeks ago the evening skies rattled with the screech of swifts, their boisterous parties skimming the rooftops and delivering sweeping curves of noise. How quiet it will seem when the last of our summer migrants has gone.

While most migrant birds have to break their journey south to refuel at favoured stopover sites, the aerial-feeding swallows and martins have the option to feed on the wing as they go. In reality, however, even these birds must break their journeys, searching out sites and habitats rich in flying insects. The birds are feeding over the river this evening because there are good numbers of flies and other insects for them to pluck from the air. The same thing was happening the other night at a friend’s stable, the insects associated with the horses and muck providing rich pickings.

It is at this time of the year that we also see swallows and martins gathering on telegraph wires or fence lines. Others may be seen at favoured roost sites, typically situated over water and within a well-established reed bed. The numbers of birds using these sites often builds up over several nights, although the roost may suddenly break up in response to the unwelcome attentions of the local sparrowhawk or passing hobby, attracted to the roost by the pickings on offer. While the sparrowhawk will remain, the hobby may well follow the martins and swallows, harassing them at other roost sites on the long journey south.

I cast my eyes upward each evening as I wander along the river, seeking out these passing migrants. Just occasionally a late swift joins them, silently feeding alongside and also stocking up reserves for the journey ahead. Some of the swallows may winter as far south as South Africa, the swifts a little further north but where the martins go is a mystery still to be answered.

The hazards of migration

The hazards of migration were brought home to me the other day by a photograph that had been sent in to our offices. The photograph showed a large number of swallows and house martins, all huddled on the narrow ledges of a building. These birds, newly arrived in the west country, had flown into bad weather and so had sought what little shelter they could find in order to sit out the heavy rain that was falling.

More widely this spring, we have seen birds held up on migration by poor weather in the Mediterranean. A blocking weather system had halted the migratory journey and forced the birds to sit out the bad weather, while using up vital reserves. At least one of the BTO’s satellite-tagged cuckoos seems likely to have succumbed to the weather. It was charted heading North on its spring migration but ran into bad weather in southern Spain; there it stayed, the signal from the tag suggesting that the bird had died.

You might think that migration was a fairly routine affair for birds, like swallows and martins, able to feed as they went along. Research suggests that even these species have to fatten up ahead of their migration, taking on reserves to help get them across barriers like the English Channel, the Mediterranean and the Sahara Desert, where feeding opportunities are presumably scarce. It is harder still for those species that do not feed on the wing and which have to stop-over to take on more ‘fuel’ at various points along their route. These ‘stop-over’ sites are important and many will be used on a traditional basis. For example, two of the BTO’s cuckoos, taking an easterly route, stopped at the Po Delta in Northern Italy on their way south. This suggests that we should look in more detail at the region to establish its role as a stop-over site for cuckoos and other migrants.

Even when the birds reach their destination they may face problems. There is a pressure to arrive early on the breeding grounds in order to get the best territories, but if you arrive too early then the weather may prove unsuitable and you may die. This year, the cold and wet spring has caught many birds out and even resident species, like tits, seem to be struggling. This time last year, we had large numbers of reed warblers and whitethroats on well-advanced nesting attempts; this year they have only just started to get going. Of course, it is only at the end of the year that we will really know just how poor the season has been. Until then, it is fingers’ crossed for some better weather.  

House Martins remain a bird of mystery

The soft, dry-sounding, ‘priit’ calls of House Martins provide the soundscape to my weekend, as dozens of birds wheel and hawk for insects in the air above me. These robust little birds have a delicate appearance, less showy than the Swallows with which they share the skies and certainly not as exuberant as the laddish parties of immature Swifts that have, by now, mostly left the country. As the martins twist and turn, taking insects above the acres of pasture over which I am walking, so their large white rumps can be seen, foreshortening the bird’s appearance even further. I’d imagine that these individuals have finished breeding and are now in the first stages of an autumn migration that will take them south over Europe and the Mediterranean, before entering North Africa to face a crossing of the Sahara.

Autumn migration begins in late August but, with some birds still feeding young in late nests, it can extend through into October. Records from our east coast bird observatories show a peak in autumn passage during September. It is amazing to think that our House Martins are part of a much bigger European population, with an estimated 90 million birds crossing the Sahara over a period of just a few weeks. This ‘super population’ of House Martins moves on a broad front, with birds initially crossing the Mediterranean from Gibraltar east to Israel. Those from Britain migrate down a route towards the western end of this wave of birds, ultimately heading for wintering grounds in West Africa.

The exact location of these wintering grounds remains something of a mystery. One of the reasons for this is that, despite the sheer number of birds involved, House Martins are not seen regularly in big numbers in Africa. This may suggest that they winter over the vast belt of tropical forest that sits across the region, hawking for insects above the canopy and out of view of researchers and birdwatchers. This lack of knowledge is in contrast with what we know about the Swallow, vast numbers of which roost communally in African reedbeds. Here the birds can be caught and ringed by researchers, who not infrequently find individuals bearing British or European rings. Fortunately, the efforts of bird ringers have at least revealed the autumn migration of our House Martins south across Europe, providing us with a better understanding of their movements than was the case back in the time of Gilbert White. White was fascinated by the autumn ‘departure’ of our swallows and martins, but was of the opinion that the birds spent the winter here in a state of torpor. Now we know different, but we still don’t know everything about House Martins.

On the move

When you think of bird migration you tend to think of summer or winter visitors, such as Swallow, House Martin or Redwing; birds that are only with us for a small part of the year. This means that many of the birds that are on the move during autumn or spring go unnoticed, simply because they happen to belong to species that you would normally see throughout the year. For instance, birds as familiar to us as Song Thrush, Chaffinch and Goldfinch all either have a migratory component to their British breeding population or to a population elsewhere within wider European range.

While most of our breeding Song Thrushes and Chaffinches are rather sedentary in their habits, some winter abroad in France, Spain, Ireland or Portugal. At the same time, Britain also receives Song Thrushes and Chaffinches arriving here to overwinter, or passing through en route further south during the autumn months. A visit to the Norfolk coast soon after dawn may well reveal some of these visitors. On Sunday morning, for example, I saw and heard good numbers of Chaffinches and smaller numbers of Song Thrushes passing overhead at Kelling Quags, together with other autumn migrants – including Brambling, Siskin and Meadow Pipit.

Both Meadow Pipit and Chaffinch are diurnal migrants (migrating during the day), and the birds I saw may well have been continuing journeys initiated in Scandinavia over recent days. Some of these birds, notably the Brambling, will have made a direct crossing of the North Sea but others, like the Chaffinch, will have avoided the crossing altogether by taking a longer route around the North Sea coast, through the Low Countries and then making a short hop across the English Channel. Such a convoluted journey will have required some nifty navigational skills along the way, given the change in compass direction needed at different stages of the journey.

Another interesting aspect of Chaffinch migration (at least for the birds leaving the northernmost part of the breeding range) is the difference in timing between males and females, and adults and young. Females Chaffinches tend to move before males (and migrate further) and the same is true of young birds, when compared with adults. In the case of the Song Thrush, individuals from different parts of the breeding range show variation in their movements; those from the northernmost populations tend to move the furthest south, with some birds even reaching North Africa. What amazes me about all these movements is not simply that such small birds can undertake such large movements but how individuals of the same species can behave so differently depending upon where they were born, what sex they are or whether they are a young bird or not.

Can you help House Martins?

The house martin is a familiar summer visitor, more suburban in habits than its country cousin the swallow. Although it once commonly nested on cliff faces, including the chalk cliffs at Hunstanton (where it last bred in 1967), the species has adapted to the opportunities provided by modern housing, choosing to place its characteristic nests under the eaves. However, it appears that the house martin is in trouble, a large-scale decline in abundance prompting researchers to place the species on the amber list of birds of conservation concern. House martin populations at the local level, for example colonies on a particular house or bridge, are known to fluctuate over time and this makes it difficult to untangle any underlying change in abundance that may be masked by such short-term fluctuations. It is clear, however, that even within Norfolk the characteristic large colonies at favoured sites have been lost. Saddlebow Bridge at King’s Lynn used to have a colony numbering 150 active nests but no longer. Nationally, the highest densities tend to occur in villages and small towns across East Anglia, so we should be concerned about the changes seen in Norfolk.

In an attempt to increase our understanding of how house martins are doing, and to add more records to Bird Atlas 2007-11, the British Trust for Ornithology (BTO) has teamed up with BBC Radio 4, to launch a house martin survey. Through the survey, researchers hope to establish where colonies of breeding house martins are to be found and to assess the size and success of individual colonies. If you have a colony nesting on your house, or are aware of one nearby, then visit the British Trust for Ornithology website (www.bto.org) to access the survey.

Despite its familiarity there is still a great deal that we do not know about the house martin. While we know that they tend to arrive here from the middle of May and depart again from September or October we know very little about exactly where they spend the winter. We know that they winter in Africa, south of the Sahara, and some 90 million house martins from Europe cross the Sahara each autumn. However, they are very rarely seen in Africa during the winter months. Unlike swallows and sand martins, they do not gather together in huge winter roosts in reedbeds or other accessible sites and it has been suggested that they actually winter above the thick jungles of central Africa. This, at least, might explain the paucity of sightings and recoveries of ringed birds. We also need to know more about the reasons for the decline seen across Europe. With luck, the new BBC/BTO project will shed some light on this particular mystery.

House MArtins may linger

The familiar house martin, with its blue-black upperparts, crisp white underparts and white rump, has had something of a mixed summer. The extended period of drought will have made it difficult for individuals to find the mud needed to build or repair nests but, at the same time, the summer warmth has helped to support an abundance of aerial prey. Many house martin pairs will be on, or have finished, their second broods and it will not be long before some begin to depart our shores. Others may be further behind and a few, well ahead of the game, will have managed to squeeze in a third brood. The recent run of cold wet weather will have limited chick growth rates, extending the development time well beyond the usual 16-22 days. This means that some youngsters may remain in the nest through into the middle of October. The presence of such lingerers often prompts phone calls to my office from worried homeowners, fearful that “their” house martins will be unable to make the journey south. Such fears are usually unfounded unless the weather deteriorates and food becomes scarce. I suppose that this highlights our attachment to these delightful birds that have chosen to nest alongside us. Although the majority of nests are now on man-made structures, especially under the eaves of houses or under bridges, house martins would have once nested in cliff faces, like those at Hunstanton.

Not everyone welcomes nesting house martins and the mess that accumulates below the nest, but it is worth remembering just how far these transequatorial migrants have come and the amount of effort they have put into building their nests. Each nest takes about 10 days to build and is made up of over 1,000 beak-sized pellets of mud. On occasion, having just completed their nest, a pair will find themselves evicted by house sparrows. These more dominant birds muscle in and take over what is, for them, an ideal nest site.

Despite the apparent familiarity, we actually know surprisingly little about house martins. Although some individuals roost within the nest during the breeding period, it is thought that most roost at altitude on the wing (like swifts). We know even less about what happens to these birds during the winter months. Even though some 300,000 have been ringed in Britain, only one has been recovered in Africa, south of the Sahara, although local reports suggest that this is where they winter. Researchers believe that house martins spend the winter months feeding on the aerial insects that gather above the rainforest canopy. Since very few people live in such areas, perhaps it is no surprise that more individuals have not been found.