There's a new "Family" report option, which lets you select sightings from a single family. This is, perhaps, most useful in conjunction with "Special reports" - you can figure out in which country you've seen the most hummingbirds, or your best big day for waterfowl, and so forth.
The "IUCN Red List" report option was broken if you'd added any sightings in extended (non-bird) taxonomies.
The AOU region now properly includes Hawaii (as well as Midway, Clipperton Island, and Johnston Atoll). The ABA region intentionally does not include Hawaii, yet - the rules committee has not yet codified this change, but will in 2017.
Remembered "total ticks" reports did not get saved properly; they will now.
Scythebill has long detected duplicate entries when importing. Previously, you only had the option of cancelling the entire import, or dropping the duplicates. Scythebill now offers another option - overwriting existing sightings with the duplicates. This is handy if you've improved the data in the original source. However, note that Scythebill only considers a sighting a duplicate if it's on the same date and is exactly the same taxon! So if you've updated a subspecies, for example, you'll end up with two sightings, since it won't look like a duplicate.
Ornitho imports should now properly handle Bean Goose imports. Absent a subspecies, they will be imported as a Tundra/Taiga Bean-Goose "spuh"; with a subspecies, they will be assigned correctly.
Observado imports will automatically set the "Photographed" bit and attach the URL of the Observado sighting if any photographs are included with that sighting.
eBird imports of the "life list" and "checklist" formats should now be more successful when you are importing from some non-English languages.
13.4.0 added a new keyboard shortcut in Enter sightings lets you navigate between the detail rows of a table when entering sightings - but it chose a keystroke on Windows (Control-Alt-Up/Down) that was already taken. It now uses Shift-Alt-Up/Down.
As always, this version of Scythebill includes further checklist improvements. In particular, I've been working through all the eBird data, looking for species omitted from Scythebill checklists. As of this release, I've checked against every country (alphabetically) from Afghanistan to Syria.