Hi Sarah -
Would an adjustable buck.boost keep outputting at the correct voltage if the inpout voltage drops? (eg one of these https://www.ebay.co.uk/itm/400W-DC-DC-Step-Up-Boost-Buck-Voltage-Converter-P...)
It would, but beware. Discharging a lead-acid battery below about 11.5V is the quickest way to permanently damage the battery. You can only really draw about 50% of the capacity of a lead-acid battery before you run the risk of permanent damage.
https://batteryuniversity.com/learn/article/lead_based_batteries
There are solutions designed for running Pis off solar, which let you shut down the Pi in a controlled matter if the power is short
https://uk.pi-supply.com/products/pijuice-standard
But you'd struggle to bridge several days of overcast skies with the battery there. OTOH the birds are more interesting in the morning and evening, so a scheduled power up/down system could work well for locusonus
Trying to run a pi 24/7 from solar systems made from secondhand junk sounds attractive, but doesn't usually end well. You can at least avoid wrecking the battery using a low-voltage disconnect, eg
http://www.reuk.co.uk/wordpress/category/low-voltage-disconnects/
Richard