Making the summer

You may have heard the old adage, 'one swallow doesn't make a summer', but what about 300 or more? Given the exceptionally wet weather recently (following a dry winter and an unusually hot early spring leading to drought), the feeling that summer might be coming is certainly a pleasant one, even though objectively the rain is welcome in farms and gardens. So, while spending the weekend at Downton for the Cuckoo Fair, although I'd been seeing small numbers for a few weeks, it was good to see a more sizeable flock of swallows (Hirundo rustica) that had just arrived...

There were about 300 in total and as you can see, when this video was taken (at about 11:00) they were mostly feeding low over the River Avon with some skimming low over the fields - traditional grazed pastures favoured by swallows. By the afternoon and evening, their feeding behaviour had changed - they were flying higher and were not following the river; instead they were moving more widely over the fields, occasionally shifting en masse as a loose flock, presumably following the patterns of movement of their insect prey.

Filming these birds with their rapid, darting flight isn't too difficult (as long as it's just a broad view that you want), but photography is another matter - group shots aren't too tricky, but focusing on individuals is another matter, hence the blurriness, though it is possible to pick out the blue head and the white flashes on the underside of the tail.

Swallows feeding over the River Avon, Downton, Wiltshire

Swallows performing close aerial passes...

Within this flock of swallows, there were a few other birds that looked different - one (black rather than blue, with a white rump-patch) was the house martin (Delichon urbicum) several of which could be found away from the main swallow flock in a nearby field. Another was smaller, fairly uniform on top, with a pale underside and dark throat-collar - the sand martin (Riparia riparia) which is also associated with this low-over-water feeding behaviour.

Among the chaos of the swallow flock, one of the three sand martins seen - and the best photo I could manage!

As well as heralding summer in the UK, migratory species such as these can be excelllent (if sometimes worrying) indicators of environmental conditions. Wintering in southern Africa, Britain's swallows face a long and hazardous journey, with perils including starvation, exhaustion and death in storms. The precise routes have not always been clear, but tracking technology has provided useful data and we now know that some cross the Sahara while others follow the west coast or fly up the Nile Valley. Rather than gaining much weight prior to migration, swallows find food en route (they fly by day and at low altitude), progressing around 200 miles (320km) each day.

It's long been known that annual weather fluctuations affect swallow populations - a cold, wet breeding season reduces insect numbers and hence chicks starve - but there are longer-term trends with populations having fallen across Europe since around 1970. Although the precise causes are uncertain, there are a number of likely reasons for this:

• The effects of climate change on swallows' African wintering grounds and migration routes. Certainly,  swallows are returning to their breeding grounds in poorer condition and laying fewer eggs than was previously the case. One factor seems likely to be the expansion of the Sahara desert,  making this already major barrier increasingly difficult to cross.
• The effects of climate change in Europe. Cold springs (including late frosts) reduce insect numbers. Similarly, very hot, dry summers cause pools to dry out, also reducing insect numbers, and as well as the risk of starvation, chicks die from heat exhaustion and dehydration.
• Land use changes across Europe may be reducing the numbers of nest sites and flying insects. Swallows tend to forage over grazed pastures (as seen at Downton), and the loss of cattle grazing has impacted on swallows in some areas.

So, there are clear concerns about swallow populations (as there are about many other migratory species) and solid evidence for population declines even if some of the causes are not fully understood. It is also a good example of the difference between 'weather' and 'climate' - in very simple terms, climate can be seen as 'average weather'. However, if you have outbuildings (garage, barn, workshop etc), there are some things you can do to encourage swallows to nest - they like to nest on dark ledges and in nooks and crannies, as these stay warm in cold weather and cool in hot weather. They can enter buildings via surprisinly small holes and need very little light (dark areas are at less risk from predators) - my favourite swallow nesting site is at Monkey World Ape Rescue Centre in Dorset where swallows nest high up inside one of the Chimpanzee viewing galleries, darting between visitors as they fly to and from the nest. To encourage swallows to nest in an outbuilding, the following should help:

• Make a small opening (minimum 50 mm high & 70 mm wide), under the eaves or simply leave a window or door open if security is not an issue
• Attach a nest platform where you would like them to be - high up, out of the reach of cats. Use flat pieces of wood to make an open-fronted box (the front should be tilted slightly upwards or have a low lip to stop the nest falling out - robin nest-box designs are sometimes used), or if you are feeling more creative, attach a sawdust-and-cement or papier-mache cup to a wooden backing plate. Block off places where you don't want the birds to nest.
• Put a plastic bag below the nest to catch droppings 
• If the weather is very hot, put an old carpet or blanket on the outside of the roof above the nest and soak it regularly with water. A couple of buckets of bathwater on such material takes several hours to dry and helps keep the temperature down inside the outbuilding.

Now, I think I've spotted a suitable nesting spot, so I'm off to rummage through my timber store to make a box. If it works, watch this space for nest photos! Until then...

Even swallows have to rest sometimes..