small guitar or banjo strings will work- get teh straight wire, not the 'wound' stuff. At a pinch i've tried ripping apart electrical wire (copper). It has worked okay but not great. Fishing line is great, but you need to prop it up with a couple of twigs or make the loops very small; that will catch bird feet, but won't strangle them, so in the name of decensy check your traps every hour or so by using binoculars.