Yesterday morning I took down the birdhouse to investigate the high/drifting weight values (the birds have not returned since last week). There was a lot of mud and moisture underneath the nest built out of moos and down feathers (please see the image below). The moisture caused the veneer on the scale plate to swell and push against the sides. This is what caused the high/drifting weight values (the actual nest by the way only weighs about 80 g). Up until now I have treated the wood only with linseed oil. The oil is environmentally friendly for the birds (it's basically vegetable oil) but apparently doesn't work too well protecting the veneer. I also noticed the gaps between the front panel and the sides of the birdhouse getting bigger. For the next version of the birdhouse I will probably add two more screws to the front/sides of the birdhouse to prevent those gaps from growing. I might even try waterproofing the birdhouse more by using some form of insulation (e.g. foam tape/silicone) between the wood panels. Less moisture on the inside will be good for the veneer (and the birds :-) ).