Methane CH4 IS lighter than air… Its molecular weight is 16. Oxygen is a diatomic molecule as is nitrogen… O2 and N2 so their weights are 32 and 28 respectively.
Use to make balloons from dry cleaning bags when I was a kid and fill them with natural gas, worked quite well.
If the cows fill that balloon with intestinal gas (farts) chicken should be careful with those lamps, methane air mixtures can be quite explosive! Am sure Steve is aware of this and will make sure the critters get home safe.. ;-)
Not to mention that fish have sex in it.