I was recently thinking a similar thing - well whether I could use my iPhone with an external mic to record birds, without the live broadcast element.
Directional mics that plug directly into the iPhone don't seem to be that impressive, and also I think you would be hard pushed to have a zoom call, while trying to point a directional mic clipped into the iPhone at a bird!
I am really looking for a system where the mic is linked wireless to the iPhone, so that I can get the mic closer to the source, but still control recording levels. I therefore started off looking for Bluetooth devices.
The first option I stumbled across was an action Bluetooth mic - the Insamic. The Instamic looks to be fun (if you can work out where to buy one!). I assume that you could place the discrete and really tiny recorder/mic close to where action may occur (say a birds song post, a feeding station, or a nest - avoiding disturbance!), and be stood 20m away listening to the audio via Bluetooth. I have not been able to find any info on the mic sensitivity, but assume it would not be so good for picking up wider environmental sounds, like a dawn chorus. I recall a birder telling me he had had fun with a waterproof speaker, throwing it on a piece of string into a marsh to temp out a flufftail. As an action mic, the Instamic Pro is apparently water proof IP7 and really tiny. I am not advocating chucking it into a marsh, but there are probably places the mic can go that other equipment can't. As the device also records at up to 96KHz 24bit (in mono or stereo versions), there so seem interesting options for being inventive. I would be interested if anyone has tried using the mic for getting up close and personal to wildlife.
The next device I cam across was the Rode Wireless Go II, which may be more versatile (but is a more expensive) option. This is not a blutooth device but is a two wireless transmitters and a receiver, the latter of which can be connected to an iPhone through a bespoke USB to lightning cable - the Rode Wireless Go II is an MFi-certified USB-C microphone . It seems to be advertised as a Vloggers mic. The transmitters have built in microphones, but as per the Instamic, I imagine that these will not be that sensitive and basically would only be of use if you can again get the mics really close to the subject. In adverts the mics are shown attached to the presenters clothes like a lavalier. Again I can't get any info on the sensitivity from the supplier. You can however plug in external mics to the transmitter through a TRS 3.5mm jack. Rode reference plugging in a lavalier (which would not be much benefit), but I am hoping that you could also plug in one of the Sennheisser K6 mics, which have their own power - obviously the 3.5mm jack will not provide phantom power, which some mics require. The interesting thing with this setup, is that the connection range is up to 200m with direct line of sight. The twin transmitters can be combined into a stereo signal, so I think in theory you could have a stereo setup for a dawn chorus, and be sat well away presenting the broadcast. I say the option is expensive, but it is much cheaper than a professional radio mic set up, and you don't have all the mucking about of applying for a frequency band for transmission, which I understand you need to do with some professional gear.
In a moment of madness, I have recently bought the Rode Wireless Go II, which is now in the post. When it arrives, I will give it a spin and report back on the pros and cons. Hopefully I will be able to record somethings with the mics hidden, but up close and personal to the source, and also get the K6 system to work with the as a remote setup. The former would be great, as I then could have a lightweight setup to use with my phone, for speculative recordings - clip the mics into the undergrowth or bushes and wait to see what comes close!.
Regards
Jon Bryant