I have a parabolic sound recorder for recording bird song.
I was recording a song thrush when in the background I heard another bird song which was very faint and I couldn’t determine if I could recognise the species
Is there any way of intensifying this faint song ? 
You can simply use the Amplify effect to boost the volume. You can select/highlight just the parts you want to boost.
If both birds are singing at the same time, you won’t be able to isolate the quiet one.
When you boost the signal, you also boost the background noise.
You can try the Noise Removal effect, but with a poor signal-to-noise ratio, sometimes “The cure can be worse than the disease.” If you try Noise Removal, use it before boosting the volume.
You can also try some Equalization to cut the mid & lower frequencies, which will mostly be noise since birds don’t make these frequencies.