Since UV’s long excursion south on 28 October, so eloquently described by Paul in the previous post, he has reverted to quite settled behaviour. His foraging strategy is still one of the most interesting aspects of his behaviour. Before discussing that, this image shows UV stopping off at the Ngalam River on his way back ‘home’ on 28 October.
UV flew low over the same small area of the river as in the image above during his ‘high flying’ outing on 19 October. On 28 October UV ventured north of the Ngalam into the Trois Marigots, a marshy haven for birds. He perched at the bottom of the Marigot Khant-Sud in the same spot as a visit about a month before he migrated in Spring. Then he flew past Toddé, where Nest 2 hatched Blue 1H was seen in November 2013. UV has also been there before, so more reacquainting himself with ‘old haunts’. He has shown no daytime interest so far in the coastal area to the south – the Langue de Barbarie National Park – which was his heartland for several months in 2015.
Back to UV’s foraging. As mentioned in previous posts seasonal fish migration and silty waters in the river mouth at this time of year will influence behaviour. Paul has created two ‘heatmaps’ of UV’s flying activity – March and October 2016.
A note of caution: all flying activity is not foraging, even when over water. But the heatmaps do reveal changes in that behaviour. Before UV left in Spring he was foraging over quite a small area relatively near the river mouth just off the north spit of the Langue de Barbarie. Now he is venturing much further offshore during part of most flights over water although the area in question is not red, but misty blue, in the heatmap. UV’s inland flying is around Lac Guembeul. He may forage there but there is less clearcut evidence – mainly he is just moving from one perch to another!
Analysing the data we can see that some days UV is looking for fish – or could be – more often than others. At this time of year there are many large fish and a good size catch, say 600g, could satisfy an osprey in winter quarters for almost a couple of days. It may be significant that the day after UV made his 182 km journey he was potentially foraging 7 or 8 times – perhaps he’d built up an appetite!
During the last few days UV has changed his strategy slightly.
UV’s more distant flight took him 4 km out to sea. The 1-2 minute fixes suggest he was looking for prey. The area just off the end of the spit appears quite productive. UV often spends several minutes there, as can be seen above. The water is shallow so silt is less of a problem. He also flew along the shoreline, a rare event since his return. Fish shoals are likely to be in different areas according to currents and wind shifts, so are they nearer the shore now?
Several times yesterday UV flew a greater distance than previously up and down the shoreline. GSM trackers with their wealth of data allow much to be learnt about probable fish movement as well as ospreys.