This week, along with our friends and conservation partners at Fundación Migres, we counted just a handful of European Honey Buzzards crossing The Straits of Gibraltar on their migratory journey. Picked out from amongst thousands of Booted and Short-toed Eagles, Eurasian Sparrowhawks, Black Storks and Griffon Vultures, each one of these gorgeous birds merited close attention to their detailed and variable plumage. Not least because at this point of the year, we never know which one will be the last we’ll see until Spring!
“Honey Season” is definitely drawing to a close and these numbers are very different to those we observed during the peak in late August / early September, when we counted over 47,000 birds crossing in just one week!
These tail-enders are the last few of the 85,000 or so that will have passed through here this autumn. This is more than the breeding population of western Europe, so it’s clear that something interesting is going on!
Spring counts of birds heading north across The Straits yield far smaller numbers – typically around 15-17,000.
Ornithologists have speculated for some time about the reason for the discrepancy in numbers. It seems – as you might imagine – that the populations that breed in north and western Europe migrate back and forth along the route that brings them through here. However in autumn, their numbers are augmented by their eastern cousins.
Research published in 2019 used satellite telemetry to follow a single adult male European Honey Buzzard, tagged in Hungary, over three years of its life. It supports the theory that many of these eastern birds follow an incredible anti-clockwise loop migration, spanning vast areas of Europe, to travel between their breeding and wintering grounds.
Once the breeding season is done for another year, the priority for these nomads is understandably getting to their wintering grounds as safely as possible. Heading to Africa via The Straits of Gibraltar is definitely the long way round, adding over 2,300 km to the trip compared to a direct route. Each autumn, the male in the study (let’s call him István!) covered an astounding 7,046km from Hungary to his favoured wintering site in North Cameroon in around 43 days.
The study also measured how much the bird was compensating for crosswinds – in other words, actively choosing his route. It revealed he worked extremely hard to stay en route to The Straits, only drifting passively for about 10% of his journey across Europe.
The advantage of this mammoth detour is to allow István to avoid the risks associated with trying to cover many miles of thermal-less water crossing the Central Mediterranean. Our 14 km sea crossing at Tarifa is nothing to these bad-ass flyers! Their compact structure, intermediate between harriers (Circus spp) and true buzzards (Buteo spp) lends itself to powered flying and leaves them somewhat less reliant on thermals than longer-winged raptors. It’s no wonder that we see them crossing in seemingly treacherous conditions, powering out across The Straits in large groups while longer-winged birds like Short-toed Eagles cling nervously to the coastal hills!
Once safely over the sea and into Africa, István spent relatively little effort compensating for wind direction while crossing The Sahara, spending over half the crossing simply drifting effortlessly over this perilous wasteland. Thanks to this and other studies, we are starting to understand that these incredible birds plan their routes not only to avoid threats but also to connect with anticipated weather conditions, such as helpful prevailing winds. Totally amazing!
István took his time on his way south to his African R&R spot. Of those 43 days, 12 of them were spent on chill-out stops! Spring however, is a different matter altogether. Speed is everything, and the race is on to return home as fast as possible and secure the most optimal breeding site.
This leads István and many like him to make a beeline (sorry!) for the breeding grounds, foregoing the relative safety of The Straits of Gibraltar and taking their chances on longer sea crossings. He flew all the way back to Hungary in just 23 days, resting for only six and slicing almost 1,500 km off his journey, crossing the Mediterranean via Tunisia’s Cap Bon Peninsula, Sicily, Italy and Albania.
Meanwhile here in The Straits, we would have been seeing almost exclusively those birds heading to north and west Europe – including France, the Netherlands, and even the UK.
For young birds though, the picture is very different. These risk-averse autumn routes are learned in later life, by following experienced adult birds. In its first year, a Honey Buzzard is effectively abandoned by its parents, which leave the breeding grounds on average 1-2 weeks earlier. It can only follow its innate sense of direction, which leads it SSW, to launch itself at whatever water body or desert it finds in its way! Only the strong – and lucky – survive. One in three young Honey Buzzards will die on their first migration while crossing the Sahara. The area of Africa where they end up wintering – and to which they will eventually become site-faithful – is largely determined by winds and chance.
After such a large investment in rearing the young Honey, this seems like some pretty harsh parental treatment! However, as always, everything makes sense in the light of evolution. Every day adults delay their own journey, and every day they would lose en route accompanying slower, less-skilled juveniles decreases their own survival chances. This more supportive parental behaviour would soon fail as a strategy. Evolution has moulded this behaviour – effectively sacrificing one in three young birds who will die on migration without guidance – to ensure that the successful adult optimises its chance of survival and lives to breed another season.
This absentee parenting strategy has certainly worked for István! He was first tagged as an adult in 2014, and is still going strong. On 7 September this year – amongst 9,499 other individuals – we observed a tagged bird crossing The Straits, who we subsequently learned had travelled to us from Hungary! I hope it was István, and I’ll be thinking of him and his epic journey ahead.