So it’s that time of year when the plants have all shriveled up and the insects are all asleep or dead, which means that amateur naturalists have only one thing to properly occupy their minds – birds. It’s a great time of year to be seeing birds of all sorts and my mind has been thinking of little else, especially of the scarcer species that tend to turn up at the onset of winter. In my native county of Sussex birds like Long-tailed ducks, Snow Buntings, Waxwings, White-fronted Geese and Scaup have arrived in small numbers. In amongst these exciting species – which despite not being common are fairly reliable each winter – proper rare birds pop up unexpectedly, birds that have arrived here more by accident than design and are often far from their natural range – these are vagrants.

In Sussex in the last month at least 8 species have been reported which could be considered vagrants, some are rarer than others but all are not where they should be at this time of year, they are lost. Which set me to wondering as to why some birds end up as vagrants in a country they never intended to visit, and why birders get so excited about finding them. A couple of good examples were found recently near me; a juvenile Rose-coloured Starling which should have been wintering in India and a young male Desert Wheatear which should be in Pakistan by now – why did they end up here?

To answer this we need to recognise that the vast majority of vagrants that are found in this country are migratory species, as opposed to sedentary/resident species, so they are not just flying off to Britain one day for no reason – most of them were heading somewhere else. The weather is perhaps the most obvious and major cause of birds ending up far off course, birds tend to fly high when migrating and are quite susceptible to getting caught up in large weather movements like storms, hurricanes and depressions. A significant source of rare vagrant birds in Britain is North America and many of these Yankee birds end up here due to tail-ends of hurricanes brushing them across the North Atlantic like so much dust.

It is very difficult to keep heading in a dead-straight line when there is a strong side-on wind blasting at you continuously, which can happen to Asian birds as they attempt to migrate from the north of the continent to the warm south – a strong and sustained easterly wind will force many Siberian birds on an acute diagonal course for the UK. We can’t blame all these foreigners on the weather though, some over-shoot their intended destination, especially on northerly migrations when perhaps the prospect of breeding gets them overexcited and they totally by accident cross half of Europe and the channel to end up looking for a mate in freezing Shetland. These might be inexperienced young birds.

Another theory is that on the odd occasion a birds navigation system screws up; perhaps if the sky is overcast so they can’t see the sun or stars or if their internal magnetic super-sense for some reason goes funky and they can’t tell which way is north or south or up or down. The fact that the majority of vagrants that end up far from home are juveniles is telling, but plenty of apparently experienced adults also get lost and not always due to the weather.

A good example is the Black-Browed Albatross that returns every year to Heligoland just over the North Sea from us when it should be in the South Pacific, it is clearly flying there on purpose and is by now a full-grown adult. This suggest it might be a ‘trailblazer’, one of the few birds whose genetic make-up is a bit different from the rest of his species, making him (probably unconsciously) find new migration routes and new food sources or even new breeding grounds which may in the future become the new normal for his species. Most such trailblazers probably end up nowhere useful and have no long-term impact, but every now and then these genetic offshoots find an unexploited niche which secures new territory for their species – this has happened in Blackcaps in Eastern Europe which now migrate to Britain in the winter rather than the Med/Africa and also in Yellow-browed Warblers which have an off-shoot population wintering in Iberia rather than southern Asia.

Then there are some vagrants that end up in the wrong place because they followed the wrong crowd (or flock), geese in particular regularly mix in with flocks of other, similar, species and follow them on their migration rather than with their own species – this brings rare North American/Arctic circle geese (such as red-breasted goose) to Britain and also happens with ducks.

So why should lost, non-native bird species get birders all in a bother when they turn up? I suppose it is rather like walking along a perfectly ordinary street in say, Scunthorpe, and coming across the actual Queen of England, it’s totally unexpected but very welcome as you didn’t think you would ever see her in the flesh. Of course the more you get out and about the chances of seeing her again are higher than if you stayed at home, likewise if you hang around likely places that the Queen might be found (Royal Ascot for example) your chances are increased again, though still relatively low, which makes it exciting.

Many of the vagrants that randomly appear in the UK are from quite far away places that not everyone will get to actually visit themselves – America, Russia, Africa, the high Arctic – and therefore not many people actually get the privilege of seeing these birds in their native and natural environment. So seeing a scruffy, dull-plumaged juvenile bird of an incredibly rare species that’s minutes away from starvation and whole oceans away from home, perched in a tree in a Tesco car park in Bristol may be the closest some birders will ever get to encountering exotic species without selling their house for a plane ticket.

There’s also the listing aspect too, birders love lists, men love lists, humans love healthy competition – we’ve gotta’ catch ’em all!