Contraceptive bait trials – our perspective.

Many of you might have seen the recent BBC news piece “Scientists design contraceptives to limit grey squirrels”. We thought we’d give you a little insight as to what we think about this trial and the impact it might have on our work in mid-Wales.

A grey squirrel against an out of focus green background. the squirrel is eating seed form the ground with its front paws.

Credit: Sam Hockaday

First of all, we should say that we support the UKSA and APHA project and we think it could be a helpful tool in the struggle to remove the invasive grey squirrel from the UK ecosystem. Many of the hopes for the project are covered in the article above, which we recommend reading. There is also a summary page on the project here and a webinar giving updates on the project on Wednesday the 13th July which you can register for here

Unfortunately, developing the “feeding boxes only they can access” is a massive challenge. The most common methods of restricting species access rely on size or weight, and if you think about your “squirrel proof” bird feeders, you’ll know they’re often not completely effective. Grey squirrels weigh anywhere from 250-650g, while red squirrels weigh generally up to 300 or 350g. On the other end of the scale, pine marten kits become active and leave their dens before they are fully grown, sometimes when they are as small as 500g. Any weighted system would need a little wiggle room to ensure 100%  prevention of non target species accessing the bait. So the feeder needs to only allow access to squirrels between 400g and 450g (as well as restricting birds and other nontarget species), which leaves so little of the population that it is not going to be effective. Size is similarly unhelpful with significant overlaps on either end of the grey squirrels range by red squirrels and pine martens.

a red squirrel perches on a tree stump and is chewing at a spruce cone in its front paws.

a red squirrel in the Mid-Wales Red Squirrel Focal Site, from one of our trail cameras.

There have been some efforts to see if coat colouration can be used for differentiating between species but, as we know, grey squirrels can have a large amount of red colouration in their coats, and red squirrels can have grey coats. As far as we know, the team have developed feeders which effectively prevent birds, mice, stoats and other mammals accessing the bait, but these would only be suitable for use in areas where there aren’t any red squirrels or pine martens. This rules out much of Wales.

It is likely that the most efficient and cost-effective use of this contraceptive bait is in combination with traditional culling methods. When trying to remove a species from an area, it is common to remove 80% of the population with the first 20% of the work, but removing that last 20% is much harder and takes 80% of the work. It might well be that in the future, for areas without red squirrels or pine martens, it will be possible to remove grey squirrels from a woodland much more efficiently by combining these methods. First knocking the population back by trapping and culling individuals, and then introducing the bait to prevent the rest from re-populating. It is possible that it will be an attractive option to those wanting a non-lethal control method The downside of this is it is likely to still require a large amount of work maintaining the feeders with contraceptive bait for upwards of five years, and during this time they will still be bark stripping trees, raiding birds nests and consuming a large amount of food.