Can I count heard-only species?

This is entirely up to you really but, whilst most birders obviously prefer to see each species, it is not always considered necessary for listing purposes, especially for something like a day-list.

One consideration for whether to count something is whether it has been positively identified. For example some species have very distinctive calls (e.g. Corn Crake, Quail, Savi's Warbler) but can be extremely difficult to see. In fact attempts to see them could be disturbing to such sensitive species, and this should be discouraged. Some other species are difficult to identify by sight alone (e.g. Marsh Tit and Willow Tit) but much easier by call, so in these cases one could consider that a seen-only record is not acceptable!

Whatever you decide, we suggest you mention it in the 'comments' for a record if you have only heard a species.