Being a small parabolic, higher frequencies can tend to be accentuated compared to lower frequencies. This creates a learning curve (like anything) in how to get the best recordings in the field.
I don't have experience of the Wildtronics Pro Mini, but for all parabolas the wave length has to be smaller that the parabola diameter to get any gain from the dish. There will therefore be no additional gain up to a frequency threshold and then increasing gain at higher frequencies. For the Pro Mini the threshold will be circa 1175Hz... for a Telinga 22 inch dish this threshold would be lower at circa 600Hz. So basically, the smaller the dish, the less use it is at recording low frequency vocalizations... the Mini Pro would not be the tool for recording many doves, many owls or say a Eurasian Bittern.
The formula for the gain of a parabolic dish includes D squared, where D is the dish diameter, so assuming all other parameters are equal, the Pro Min will only produce circa 28% the of gain produced by the larger Telinga dish. Obviously there is a compromise over size and easy of use verses gain levels (and presumably a 22 inch dish is seen as the upper end of what can be managed in the field, as this seems to now be the standard full-sized dish). The sensitivity of the mics will also play a part, but gain from a dish should be without self noise and is directional, so to some extent gain from a dish is better than having a higher sensitivity mic to create the same signal strength (or applying additional gain with the recorder). In the examples I have seen of other small dishes, they are a definite improvement over open mics for high frequency vocalization, but if you are targeting more distant birds, then a larger dish will likely provide a better recording.
All dishes create increasing gain at increasing frequencies. The argument is that this compensates for the fact that high frequency sound dissipates quicker over distance than lower frequency sound. The parabola therefore 'restores' the frequency balance, recreating 'nearness'. This is all well and good, but it does mean that a recording with a parabola will not sound the same as it does to the recordists ear... the frequency balance will be different... and if we are familiar with hearing Lesser Whitethroats at c20m, then hearing a recording, which creates the impression it was only 5m away, may well sound strange. This is the argument over whether to record a 'bird's ear' or 'human ear' recording, which was raised in the first Sound Approach book.... and another reason some people prefer not to use parabolas.
...and as CMB states, dishes will pick up noises not undetected by the human ear. Goodness knows why, but when I first got a parabola I pointed it at a flying Wood Pigeon, circa 30m away, and was stunned by the amount of wing noise!'
