Slope Soaring and Ridge Riding

Migration in Detail - (Part 2)

In the previous article in this series, we looked at one of the energy-saving flight modifications used by ospreys and other raptors during their migrations. Like human glider pilots, ospreys have developed a whole range of techniques that can be used - either singly or in combination – to gain maximum advantage from the prevailing conditions.

Using data from the latest GSM-type tracking units, which can log flight parameters at intervals of minutes - rather than hours, as was typical of the older UHF devices – we can get a much clearer view of what these birds are doing and how they are doing it...

In November 2014, adult male osprey “Tero” was flying south-west down the Arabian Peninsula. After a major diversion to avoid adverse weather south of Iraq, he had reached the Jabal Tuwayq - a long north-south escarpment that marks the eastern boundary of the Asir Highlands in Saudi Arabia. The winds were light but slightly against him. But Tero was able to use the overall rising air current caused by the gentle terrain gradient in a method known to pilots as “slope soaring”

 

As with the “crosswind tacking” technique, this allowed him to gain altitude by turning UP the slope, and then maintain course progress by flying down it at a shallower angle. The advantage of this system is that it works for almost any wind direction that is at a greater angle to the line of slope than 30 degrees.

"Blue 7H" (Image: Joanna Dailey)

A variant of this flight mode is what I've chosen to call “ridge riding”. It is an adaptation to more complex upland terrain where there are many changes of elevation, with steep-sided river valleys and hill crests. And the example chosen this time features “Blue 7H” - a female first-time migrant from nest #2 at Kielder Forest. Blue 7H provides the possible answer to a question that came up on one of the discussion groups, which (in summary) was:

“Do juvenile birds have the innate (instinctive) ability to use these energy-saving techniques, or do they have to learn them as adults?”

Only seven days after leaving her natal nest, 7H had reached the Galicia region of northwest Spain. Crossing this mountainous and forested landscape, she took advantage of local up-currents along the windward side of ridges to maintain the necessary height and made good progress southward and into Portugal. It seems like even a young bird of prey comes equipped with the full repertoire of flight, and only needs to add a modicum of practice. This confirms visual observations of other migratory species on their first migrations.

In the final part of this series, we will look at some other flight modes that are used by ospreys, and see how their anatomy and wing layout influences what they are able to do.

Wildlifewriter acknowledges the use of tracking data supplied by the Natural History Museum of Finland (Luonnontieteellinen Keskusmuseo)and the Finnish Osprey Foundation, and data and images by Forestry Commission England.

Links:

LUOMUS: http://www.luomus.fi/en

Kielder Osprey Blog: https://kielderospreys.wordpress.com/