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.