Osprey Flight Speeds: Are females faster than males?

Monty chases Glesni, Cors Dyfi 2013
(Montgomeryshire Wildlife Trust)

Background 

“How fast can an osprey fly?” is a topic that often comes up when these birds are being discussed. There is, of course, no straightforward answer to this question: all birds have a range of flying speeds, dependent on what they happen to be doing at any given time. Nor is it easy to establish a “maximum speed record” for any species of bird – unless they are pursuing or being pursued, there is no real reason for a bird to fly flat-out over any distance since this would be wasteful of energy. Several attempts have been made to establish a general rate of progress for migrating ospreys, using time intervals between sightings, or by point-to-point measurements from satellite tracking. None of these methods are really satisfactory, since there are so many variables involved. The most significant contributions have come from Sweden, by Alerstam, Kjellen and others. [1] 

In 2006, Lorna Shaw (then an undergraduate at Nottingham University) conducted a study [2] of migrating ospreys pandion haliaetus using data from the Rutland Water translocation project. Taking a sample of 21 individual birds, Shaw concluded that female ospreys might fly slightly faster than males, although the variance was not found to be statistically significant. Given the state of tracking technology at the time, this was to be expected.

We decided to address this question by using a different approach. The latest generation of GSM-type tracking devices record flight speeds directly, and at much shorter intervals. We selected two first-time migrants (sub-adult), one female and one male, that had been hatched (from different nests) at Kielder Forest in the north of England. The birds are hereinafter identified as “UV” (m) and “7H” (f) from their leg ring codes. Both ospreys had been weighed and measured by Forestry Commission staff during the ringing process; they were found to be in good health and well-nourished, and so can be considered representative of the general population. 

Methods

The raw tracking data for both birds was obtained courtesy of ForestryCommission England and loaded into a relational database system. (See sidebar) The object of the study was to obtain an analysis of ALL flight movements since fledging, from September to end December 2014. The hypothesis that female ospreys generally fly faster than males was to be tested. 

The birds had been fitted with 30gm GPS/GSM tracking units manufactured by Microwave Technologies Inc, Mass, USA. These devices output animal movement speeds in knots (nautical miles per hour) and this is the unit referred to throughout, unless otherwise stated. Speeds are “over the ground” in all cases. Since we were interested in all aspects of flight – local, foraging, and migration – these were not segregated. It is recognised that the wind is a major element in bird flight and no attempt has been made to correct for its effects, nor would this have been practicable. 

Record selection and processing 

The initial data tables contained over 20,000 samples for each bird. We filtered out all entries where the speed was recorded as zero, and also deleted all records that were obvious GPS system or transmission errors. We also removed any other entries that appeared to be anomalous for any reason, even if the speeds shown were within acceptable criteria. (In the case of 7H, I manually deleted those records where she was known to be taking a lift on ships, as this might have given her an unfair advantage!)

After this preparation, the remaining sample sizes were: n=9192 for UV and n=9365 for 7H.

To summarise the data, a range of speed classes were selected to construct a univariate frequency distribution. The nominated range was 1-60 knots, in increments of 5. (In the event, no valid speeds in excess of 55 knots were observed.) Corrections were made for the slightly different overall sample sizes, and this resulted in the table and graph following...

Speed Classes (Kts)	Range UV	Range 7H	% Range UV	% Range 7H
1-5	243	255	2.644	2.723
5-10	597	615	6.495	6.567
10-15	1740	1792	18.930	19.135
15-20	2208	2248	24.021	24.004
20-25	2274	2296	24.739	24.517
25-30	1201	1222	13.066	13.049
30-35	634	641	6.897	6.845
35-40	209	210	2.274	2.242
40-45	74	74	0.805	0.790
45-50	10	10	0.109	0.107
50-55	2	2	0.022	0.021

[Click for larger]

Conclusion 

In their daily behaviour, the most common flying speed for young ospreys is in the range 15-25 knots (28-46 Km/h). Velocities higher than this are not uncommon, and wind assistance during migratory passages is probably a major factor.

Although the male bird appeared to demonstrate a preference for slightly higher speeds in the medium range and lower speeds elsewhere, an ANOVA variance analysis of the data indicates that this is not statistically significant. We conclude that there is no effective difference in flying speeds between the sexes.

The birds studied had hatched in the same natal area and commenced their annual migration within a few days of each other. Although they took different routes southwards, and had long stopovers at different locations, these routes and locations (UV in SW Portugal and 7H in NW Morocco) would have had generally similar weather patterns and wind conditions during the period being studied. For this reason, the comparisons between the data sets are probably as valid as could reasonably be expected in the wild.

Links:

[1] "Factors affecting the autumn migration of Ospreys" Shaw L, Rutland water website: http://www.zen88810.zen.co.uk/ROspreys%20site/Satellite%20analysis.htm

[2]  "Timing and speed of migration in male, female and juvenile ospreys Pandion haliaetus between Sweden and Africa as revealed by field observations, radar and satellite tracking." Kjellen N, Hake M, & Alerstam T. (2001 Journal of Avian Biology 32, 57-67.

Sebkhets of the Sahara

Geography Lessons from Ospreys #438

Location: Assouerd Province, Western Sahara

Dama Gazelles (Image: W.A.Z.A. Conservation)

The western coastal margin of the Sahara Desert is a region with almost no rainfall. To our eyes, it appears desolate and lifeless – and yet there IS wildlife here.

Among the dunes and wind-scoured rocky plains, isolated pockets of hardy vegetation grow, flower and set seed. The seeds attract sand grouse and roving bands of migrant finches. Dainty gazelles known as “Mhorro” (Nanger dama mhorr) which are critically endangered elsewhere in the Sahel, still graze there, and the almost-mythical white antelopes (Addax nasomaculatus) - one of the rarest hoofed animals in the world and until recently thought to be extinct in the wild, are also to be found... if you know where to look.

The reason for all this is the presence of sebkhets.

A sebkhet is (in this context) a low-lying area where groundwater from aquifers below the desert seeps to the surface. They can be brackish or can contain fresh water, depending on the local geology. The mendicant camel-drivers have always known about them and some of the larger, more permanent ones have Arabic names. Others are smaller: they can last for only a few years or even months, before the desert wind covers them with sand again and exposes some other sebkhet elsewhere.

Addax nasomaculatus (Image: wildaddax.org)

Ground-reflectance images from satellites, together with LIDAR measurements, have enabled scientists to map the distribution and extent of the desert sebkhets. There are many more of these features than was originally thought, and it is now clear that they are the secret of how large grazing mammals have been able to survive in a place where none should be. At the Parc National de Safia, a closed reserve as been established where the Mhorro and Addax are protected. Their numbers and gene pool have been bolstered with captive-bred animals from European zoos, and the last count was up 23% overall since the project started.

In UV's area, there is a permanent sebkhet and several small un-named ones. Perhaps it isn't such a dull place for an osprey after all!

[Sources:]

Directory of African Wetlands, R. H. Hughes, J. S. Hughes, G. M. Bernacsek
Antelopes: West and Central Africa – action plan, R. East, ed. (WWF)

Wild addax website: http://www.wildaddax.org/description.html

Review: "Ospreys in Wales" by Emyr Evans

This is not your average bird book. Ospreys in Wales – the First Ten Years by Emyr Evans recounts a remarkable conservation success story – but it is far more than that. At its very core, this is a book about faith and inspiration, and about how the combination of these rather abstract concepts brought about the re-establishment in Wales of these rare and charismatic birds of prey.

It is at once a history, a memoir - and a thriller, with all the attributes required: excitement, jeopardy, sex and violence, rivalry, betrayal, tragedy - and flounders. (Though not necessarily in that order.) After an erudite - and lavishly illustrated - introduction has set the scene, Evans expands and develops the story. He writes in an easy, conversational style that seamlessly draws the various locations and events together. Eventually the birds themselves take centre stage, as larger-than-life characters in their own dynastic saga – the englynion of ospreys.

There is plenty here to delight the enthusiast, but the casual reader too is in safe hands. Evans successfully avoids the twin pitfalls of anthropomorphism on the one hand, and dry ascetics on the other. Instead, we are treated to a narrative that carries us along into the fascinating world of ospreys. Details and anecdotes abound, to be sure, but they are presented lightly and with the author's own passion for his subject shining through. Nor is his sense of humour (familiar to all of us who know him) absent, though here it is suitably reined in.

Emyr Evans is a behavioural ecologist by training, but he is also a photographer of rare skill with a sure eye for composition and effect. Ospreys in Wales would be significant for its text alone but the crowning glory of this hardback edition is the stunning collection of colour plates that adorn and inform every page. Drawing on his own library - as well as contributions from professional Andy Rouse and others - Evans shows us why tens of thousands of people around the world now follow every move that these birds make.

This is a true expert's story, expertly and compellingly told. Highly recommended.

"Ospreys in Wales - the First Ten Years" E. Evans (2014) ISBN: 0993099009

Amazon: http://www.amazon.co.uk/Ospreys-Wales-First-Ten-Years/dp/0993099009

Website: http://www.ospreysinwales.com/

Ghost Town

Geography lessons from Ospreys #437

Location: “Puntillas de las Raimas”, Cintra, Western Sahara, N 23.08, W 16.20

Kielder osprey “Blue UV” arrived at the Gulf of Cintra on 11^th December 2014 after an epic 2000 km migration flight from his long stopover in southern Portugal. We were intrigued – few if any tracked birds have ever visited this remote and disputed spot on the edge of the Sahara Desert. As UV looked around his new discovery, I did the same thing on Google Earth. I knew almost nothing about it. Were there any signs of human habitation around the place? There were not.

But there used to be...

Separated in time by nine years, these two aerial photographs of the same location show that there was once a thriving fishing community here at Las Raimas.  Now the hundreds of boats - rough-built inshore craft known as piroques - are gone, and the shanty town that housed their owners and crews is almost buried under the shifting coastal sand dunes. The bay is in a prime position for fishing (and for migrating ospreys) located as it is in the middle of one of the most productive continental shelves on the entire planet. The lee side of the bay is an ideal haul-out for these shallow-draft boats as it is sheltered from the prevailing northerly winds and the powerful Atlantic surf. So what caused these people to abandon their village during this period?

It could have been the local security situation: Western Sahara (formerly Spanish Sahara) has seen sporadic armed conflict for years, victim to the rivalries of its more powerful neighbours: Morocco, Mauritania and Algeria. All of them have coveted its natural resources, but none of them have done its people much good.

Perhaps it was the encroachment of the desert itself, or a failure of the (never reliable) water supply, or the attractions of a less-hazardous way of life in some other line of work. But I don't think it was any of these factors...

The German-registered super trawler "Maartje Theadora" operating off the
coast of Mauritania, 2013. (Greenpeace)

In recent years, the traditional artisan fishing industry in west Africa has been taking a hammering. Locally-owned fish processing stations have closed, boats have been laid up, whole coastal communities have been displaced. The hammering has been administered by large foreign factory trawlers from Europe, from Russia, and now from the Far East as well. Operating around the clock - and on a netting scale that the piroques cannot match - they can take out in one single day, more tonnage than all the fishermen of Cintra would have harvested in their entire season.

The big trawlers need nothing from the land, except fuel oil. They process and refrigerate their own catch, and do not need to sell it at local markets. They respect neither national agreements nor quotas, nor the boundaries of marine reserves. The discarded by-catch includes every creature that swims, and it all goes back over the side – dead. For the countries affected, it's an economic and environmental disaster. But down along the same coast, one man has had enough.

Haïdar el Ali is Senegal's pugnacious and mercurial Minister for Fisheries. Since moving from Environment - where last year he took on Big Timber with devastating effect – he has revoked 29 foreign operating licences, arrested and detained the Russian trawler Oleg Naydenov, and is personally overseeing regeneration projects among his country's artisan fishing communities. You can read more about his exploits HERE.

But what then for Las Raimas? Its people may have moved elsewhere, but they will have taken their disturbance and their pollution elsewhere, too. The lagoon at the north side of the bay - known as the Bajo Tortugo ('little tortoise', after the curiously-domed sandbar that guards its seaward entrance) is now clean and quiet. At least one osprey has come there instead for a visit. And he has come to a very remarkable place, because the Gulf of Cintra is not just blue ripples and an empty beach: its warm shallow waters are thought to be a nursery area for rare marine life and mammals, including Risso's dolphins and the critically-endangered Mediterranean monk seal. With suitable research and protected status, it could be as important to the eastern Atlantic ocean as the Sea of Cortez is to the Pacific.

 I didn't know any of this before UV arrived in the area, but I do now. He is doing a great job.

LINKS:
Forestry Commission England, Kielder osprey blog: https://kielderospreys.wordpress.com/
EU fisheries policy in West Africa (Oli Brown 2009) http://hdr.undp.org/sites/default/files/hdr2005_oli_brown_29.pdf

Biodiversity - Atlantic coastal desert: (WWF) http://www.worldwildlife.org/ecoregions/pa1304

The Migration Merry-go-Round

Migration in Detail - (Part 3)

Way back in 1999, I knew even less about ospreys than I do now. But I did know some things about birds and I had completed my foundation course in Meteorology, so when the Rutland translocation project came to my attention, I – and many other enthusiasts – followed its progress with increasing fascination. Some of the translocated juveniles were fitted with tracking devices. In those days, the units were primitive and highly experimental: they used ARGOS doppler signalling to calculate the positional fixes (which we now know to be a less-than-ideal method.) The trackers were as unreliable and short-lived as a politician's election promises – and only marginally more accurate. But they worked.

When one of the birds, a male tagged as R03(1999), showed some unexpected course deviations over the Sahara Desert, I suggested that he was avoiding a formation of adverse weather systems (which I had detected on weather satellite images) that had developed to the south-west of his position, and that these might affect his ability to use soaring flight while over the desert. This suggestion was put to the Men Who Knew A Lot About Ospreys at the next group meeting. It was not favourably received.

“Ospreys on migration use thermals very little, if at all” was the conclusion of the MWKALAO, although they didn't offer any alternative theory to explain the bird's behaviour. Being a humble and uninformed amateur, I accepted this as being the expert view...

Until now

When the published track from Finnish osprey “Helena” came in for early October 2014, it seemed to show that here again was a bird using soaring techniques while over the desert.

Unlike the examples I produced for previous articles in this series, Helena's sample is not “slope soaring” or any other kind of terrain-following flight. There are no slopes at this point, no escarpments or ridges to provide an updraught. To me, these movements looked very like direct thermal altitude modification. But it was not quite conclusive. To be absolutely certain, I needed to get my hands on the actual data files for an osprey flying over Europe (where there are lots of GSM cell towers) so that the recorded level of detail was as high as could be possible with this new technology. 

Joanna Dailey, who volunteers with the Forestry Commission osprey conservation project at

Blue VV at nest Aug 2014. Blue UV in the background
(Image: Forestry Commission England)

Kielder Forest, arranged for me to get authorised copies of their files for three juveniles. They were carrying GSM trackers and all were operating correctly. The hunt was on.

Two of Kielder's birds followed a mainly coastal and/or over-sea route for their migration. But the third, tagged as “Blue VV”, migrated over land and through central Spain. With light winds and hot sun in late summer, this is prime territory for the formation of thermals. Each day I had been carefully saving the weather charts for this region and these told me exactly when and where to look for the evidence. And little VV did not disappoint me.

Even position fixes at 70 seconds apart does not conclusively prove that spiralling flight has taken place – but adding in the direction of travel at each data point does. During this afternoon flight, there are eight other examples of the same behaviour, roughly at even time intervals.

So... were the Men Who Knew A Lot About Ospreys wrong, fifteen years ago? Well, not really, but they may have been misled by the early technology which - given the comparatively low number of cumulative fixes - seems to show birds flying in reassuringly straight lines across the broader landscape, when in fact they do nothing of the sort. As I have tried to show in this series, we are still learning things about the minutiae of bird migration...

… and there's a lot more yet to be discovered.

In the final part of this series, we will consider the physiology and layout of raptor wings, and see how evolution has adapted different species to have the appropriate “equipment” for their varying lifestyles.

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:

Rutland Water Translocation History: http://www.ospreys.org.uk/osprey-facts/the-translocation-project/

Kielder Osprey Blog (Joanna Dailey): https://kielderospreys.wordpress.com/

Bird's Soaring flight (Technical): http://arxiv.org/ftp/arxiv/papers/1012/1012.0434.pdf