Your question requires a bit of clarification. When you talk about estimating SPL of bird calls, do you mean the received level (RL) of the calls at your microphone, or the source level (SL) as the bird produced the sound? SLs are typically given in dB SPL (or dB re 20 µPa) at a distance of 1 m from a source.
If you want SLs you first need to measure (or estimate) the RL and then apply an adjustment for the distance between the bird and your microphone, since RL falls off logarithmically with distance from the source.
In order to get actual RLs from a recording, you need to have calibrated (or more properly “characterized”) recording equipment so that you know the scaling relationship between the digital sample values in the audio recording and actual sound pressure at the microphone, measured in micropascals (µPa). There are two approaches commonly used to obtain this calibration info. The first “stepwise” calibration approach relies on knowing the way in which the signal is transformed at three stages: (1) the microphone, which converts pressure variations into voltage variations, (2) the preamplifier, which amplifies the weak electrical signal from the mic, and (3) the digitizer (analog-to-digital converter) which converts the continuous voltage waveform into a series of digital audio samples (typically 16- or 24-bit integers).
The second approach is to treat the entire recording system (mic + preamp + digitizer) as a “black box” and just determine what the scaling relationship is between the audio sample values and the sound pressure (in µPa) at the microphone. Usually this is done by making a recording of one or more calibration signals (usually pure tones) while also measuring the level at the microphone with a sound level meter.
These approaches are summarized in the attached Powerpoint slides.
Beginning in Raven Pro 1.5 (currently in a beta version) you can enter calibration information if you have it available to get true SPL values out from the measurements. Without that calibration info, dB measurements in Raven can be used for making relative comparisons between sounds in the same recording (or in different recordings if they were made with the same equipment and gain settings), but not to infer actual dB SPL.
One caveat to keep in mind is that if you change gain settings during a recording or between recordings, you need to know what the gain setting was at every time, and what this gain setting corresponds to in terms of dB of amplification.