The Grey Squirrel may appear to be quite cute to you and I, but to a hole nesting bird it is nestling KILLER No 1. This intelligent creature goes about it's business quite methodically. Once it has discerned that there are chicks within the nest box, it simply tries to reach through the hole entrance with a forearm and pull a chick out, then eat it, returning for the rest in turn as hunger dictates. If the chicks prove to be out of reach then Plan B for our little furry fiend is the tried and tested method of chewing his way in using the finest set of teeth that nature could have possibly provided for this very purpose. He usually starts at the entrance hole by enlarging it in a downward direction. The picture left was taken of just one of three destroyed wooden nest boxes at the Marchwiel Marsh Reserve Nature, near Wrexham, showing grim evidence of the demise of the resident brood.
The clear evidence displayed above demonstrates just how easy it is for a squirrel (woodpeckers are quite proficient at chiselling their way in also) to break into a wooden nest box.
It makes only a small-time difference if the nest box is made of softwood or plywood. There are many “pretty” wooden nest boxes on the market but they are ALL avian death traps. I beseech my readers not to put the birds at risk by buying a wooden nest box. I will now show you, step by step, what we do to prevent a grey squirrel from gaining entry into one of our nest boxes. At left is a picture of one of our typical Glass Reinforced Plastic (fibreglass) nest boxes. A squirrel cannot chew through fibreglass without breaking its teeth, period. So that takes care of five sides and leaves only the wooden front. As mentioned above squirrels usually start chewing their way in by way of the entrance hole and where it comes to our nest boxes the edges are not an option principally because GRP circumscribes the wooden front. Moreover, there is a laminated side strut to which the wooden front is fastened and the overhanging fibreglass canopy makes it nigh on impossible for this dexterous little fiend to even cling on underneath it. In days of old they had a Portcullis to stop the bad guys breaking in. In modern times, we use a little high tech unashamedly. Pictured left shows the wooden fronts of our nest boxes with a routered rebated ring of 2 millimetres depth. The below left picture shows the same wooden fronts with laser cut stainless steel (we don't do rust) rings stuck in.
The next step is to glue on our trade mark hardwood entrance hole rings, in fact replicas of natural tree holes which so attract the birds. This effectively hides the laser cut ring between the wooden front and the wooden ring, see left. Just to be ultra-secure we then put three countersunk screws through from the inside which transit the steel ring into the entrance hole ring. We ensure that one of the screws is plumb centre bottom, in other words precisely where a squirrel would attempt to start chewing into the entrance ring. He starts with the sharp end of a screw! See below left. Now you might be asking, “why not simply screw a square steel plate with a round hole in it over the entrance hole of the wooden front?”
My answer is that the birds need something to grip to with their claws around the entrance hole. We are unapologetic that our nest boxes are well thought through and we use any means, old or modern, to achieve the superlative nest box.
No doubt you recall the squirrel Plan A, namely the arm through the entrance hole, grab a chick and that takes care of today's lunch. We install a small platform inside the chamfered entrance hole which prevents the squirrel articulating his forearm downwards. Checkmate. See below.
I am very grateful to very experienced contributors to my understanding in these matters, particularly Chris de Feu both directly and with his book, Nest box Guide from the BTO. Also to Ian Spence and Giles Pepper from the Welsh Ornithological Society. Any further ideas toward bird security is very welcome, please do get in touch with good ideas.