Posted: 01/05/2014 at 18:45
This one I may know. Mice or rats I think. Maybe squirrels if the poly-tunnel is well secured all but a high vent. I notice that you have some affiliation with cats. If you cover the plants/seed trays with chicken wire (against possible mistakes re: litter tray look alike) and put the cat/cats in there for a few hours each week on a shady day, the smell of the proximity of them is often enough to deter a rodent.
I have read lots of complaints about cats in gardens, but out in the countryside we know exactly what cats are for (beside being some of our best furry friends). They prevent rodents from getting into places where we are storing food, seeds, grain, bulbs etc. Keeping the cats off growing plants (and bird feeding areas for that matter) is no problem, you just put wire over the treys, tables and beds. I suppose they could be more of a problem to some people than the rodents in towns and suburbs were keeping things looking tidy and containing children in small areas is important, but I think that out in the countryside they are the most humane and effective solution (short of a ferret perhaps).
For those concerned, since fortifying my bird table I have had one bird death this spring, which related to some accidentally split bird food - rather sad but not too bad a record considering there are 11 cats cohabiting here. No mice or rats in the house or sheds - not a whisker.