Waiting for news

In our last update we reported that the satellite-tagged juvenile female honey buzzard from Moray had reached the River Niger in Mali after an 11 day crossing of the Sahara. The last data transmission, at 10:20 on 11th October, showed that she had crossed the river and was perched on the south bank for 30 minutes.

The honey buzzard’s migration to Mali
She was perched on the south bank of the River Niger on the morning of 11th October

Unfortunately, we have received no further transmission from the satellite tag since, but there are reasons to remain optimistic. The satellite tag continually logs and stores data, and then transmits it at pre-determined intervals using the Global System for Mobile Communication (GSM) network. This means that in order to transmit data that has been collected, the bird must be in an area with GSM coverage (i.e. within range of a mobile phone mast) at the time of the scheduled data transmission. If it is not, then the tag continues to log and store data and attempts to transmit again at the next scheduled time. It was for this reason that we did not received any scheduled updates as the bird crossed the Sahara between northern Algeria and the River Niger in Mali. It was only once the bird arrived in an area of GSM coverage close to the River Niger, that we finally received details of its Sahara crossing.

After reaching the River Niger in Mali on 10th October, we expected the bird to continue south the next day. We received the scheduled transmission at 10:20 that morning, but not the next one at 17:20 that evening. This suggests that the bird continued south sometime after 10:20 and then roosted in an area with no GSM coverage. Looking at a map of the sparse GSM coverage in Mali (see below), and taking into consideration that the tag was performing normally and battery voltage was high, this certainly seems a plausible explanation.

GSM coverage (purple shading) is very patchy in Mali, and probably explains the lack of recent data (source www.gsma.com)

The question is, where is she now? Based on her south-westerly trajectory between 6th and 10th October, it is likely that after leaving the River Niger on 11th, she continued on a south-westerly heading towards the tropical forests of Guinea, Ivory Coast, Liberia or Burkina Faso which she could conceivably have reached within three-four days, based on the average daily distance she had covered while crossing the Sahara (250 km per day). One of two English juveniles we tracked in 2003 wintered in the coastal forests of Liberia, while Vespa, a Scottish juvenile male we tracked in 2009, initially migrated to Nigeria, before heading west to Liberia. It is quite possible that she has remained out of GSM coverage during this time, and is now in the tropical forests favoured by honey buzzards during the winter, still out of GSM range at the scheduled transmission times.

The honey buzzard (white arrow) appeared to be heading towards the tropical forests in West Africa where other satellite tagged juveniles from Scotland and England have wintered (yellow arrows)

There is a precedent for these kind of data gaps from this region. A Finnish satellite tagged Osprey, Seija, wintered near Lac de Buyo in the southern part of the Ivory Coast during 2014/15, but the Finnish researchers received no data transmissions after 5th October 2014 when the bird was in the south of Burkina Faso, en route to the Ivory Coast. It was not until she flew back into GSM range on her northward migration the following spring that they finally received the missing data, covering the remainder of her autumn migration, and the whole of the winter. It is quite conceivable therefore that, assuming the honey buzzard is still alive – which we think she may well be – that we will not receive any data for some months yet, particularly as she is likely to spend the winter in dense forest. Our hope is that, unlike an adult osprey such as Seija, the young honey buzzard is unlikely to be sedentary for the whole winter, and so may move into an area of GSM coverage at some point. It is also possible, of course, that she has remained out of GSM coverage in Mali.

We will, of course, provide an update as soon as we receive news from the transmitter.