Satellite tracking – the live map

Frequently asked questions

• Why hasn't a fix been received in the last few days?

  The tags have tiny batteries that are charged by sunlight by means of a solar panel. The length of time it takes to charge depends on the strength of the sunlight so in bad weather it may take days to get enough power to communicate with the satellite. If the bird is feeding in shaded areas or if feathers covers the solar panel it can also take a long time to charge the battery. Also on each satellite pass, the satellite needs to get a number of messages from the tag in order to be able to work out where it is. If it doesn’t get enough messages we don’t get a fix.

• Why isn't the line between sites a smooth line, as aren't the birds flying?

  We only get occasional fixes so the line is a straight one between each fix. It may be hours or days between fixes. So the tracks are the approximate, not necessarily, the exact route taken by each bird.

• How accurate are the fixes?

  The accuracy varies with a number of factors. Most importance of these is from electrical signal interference from industry. Each fix has an error estimate which will vary from a couple of hundred metres to as much as one hundred kilometres. The accuracy depends on the number of messages each satellite gets as it passes overhead.

• How big are the tags? Doesn't this affect the birds' flight?

  The total weight of the tag when attached to the bird is 1.9 grams but the tags are only designed to stay on for a few months. Shorebirds that migrate long distances and can double their body weight prior to migration so we aimed for the tag to be under 5.5% of their normal body weight. We undertook trials on attaching similar tags to six captive bred dunlin and could find no effect of the tag.

• How are the tags attached to the birds?

  We glued the tag to the back feathers using superglue. The tag will fall off after a few months and the feathers will regrow. We did not want to risk a bird carrying a tag for life even if it had stopped transmitting so we chose the gluing method.

• Why are the tags only expected to last a few months?

  The tags are designed to last several years but the tag will fall off after a few months when the back feathers are moulted and replaced. The very longest they could stay on is until next autumn when they change all their feathers.

• How do the tags transmit data?

  The satellite tags upload short duration radio messages (of less than one second) to Argos instruments on satellites that pass overhead at an altitude of 850 km. Due to the Doppler effect, a shift in wavelength of these radio transmissions occurs as the satellite passes overhead and this allows the ground-based part of the satellite system to calculate the positions of the tag and the bird carrying it. The transmission of each message to the satellite takes less than one second.

• Are the maps really live? How do you do this?

  Our IT whizz, Robin Jones, has developed a clever system which downloads data from the Argos data servers every ten minutes. These data are automatically imported into our telemetry database and a message is sent to the tagging team members to alert them of the new fix. Simultaneously, the tracking map is automatically updated from the database. So any fix on the map is bang up to date. It’s amazing!