The Birds in Your Garden

On the excuse that House Martins and Cuckoos could be garden birds, I’m going off-piste to talk about a topic that fascinates me – tracking birds, whether when migrating or just moving around feeding.

One of the BTO’s aims is to push the frontiers of technology to learn more about bird behaviour, and whilst trackers of one kind or another have been around for more than 20 years they are still evolving and miniaturising at pace.

Trackers weigh from 0.3 gram up to anything a bird as big as a gull can easily carry. Some can transmit their position to a remote receiver, whilst others require the bird to be recaptured or recovered, if dead. All require the bird to be caught for installation purposes, which can in itself pose a number of problems!

cuckooPhoto: BTO

The smallest trackers are geolocators which are a combination of a light sensor, a memory chip and a clock, allpowered by a battery. These tiny devices use light levels to plot local day length which, when combined with the sun’s position above the horizon,gives an estimate of latitude and longitude to within about 150 km. Good enough on a 5,000 km journey. Heavy cloud cover or the bird being in shade can lead to inaccuracies, but over a period will even out. Changes in light levels can also be used to tell when a bird moves onto or off its nest, which is useful when monitoring breeding.

Next up in size and accuracy are tags which use GPS. These weigh from about a gram upwards, the smaller ones only storing data internally for later analysis, whilst the larger ones can transmit their data. The accuracy of these systems can be anything down to 20 metres depending on how sophisticated they are. Mass production has made these tags cheap enough to allow large samples of birds to be tagged at once, vastly increasing the amount of information collected.

Finally, there are the satellite and mobile-phone-based trackers used on larger birds such as Cuckoos and gulls,which can transmit their data. Those using mobile phone networks are larger and heavier at 12 grams or more as they need much more power, but as they have lower transmission coststhey are often the favourites. Some satellite tags use solar panels to charge their batteries and can cost as much as £2000 each, as well as incurring high transmission costs.

The choice of which tag to use for which purpose depends on many factors: the size of the bird, overall cost, ease of recapture etc. All this data has many uses:locating food sourcesduring migration, looking at flight patternsto work out likely conflicts when siting off-shore wind turbines, discovering breeding sites and many, many more. It’s exciting stuff and we’re learning a lot. If you want to know more, or follow the cuckoos on their way back here, look on the BTO website.

If you find the lives of our garden birds to be of interest, and would like to join in and count the feathered occupants of your garden, please contact me or visit the BTO Garden BirdWatch website( If you know of an organisation not a million miles from York which would like a talk on garden birds call: Mike Gray 07596 366342 or gbwmike