Hello millipede and welcome to the WAB forum.
For recording events and taking notes while in the Field, I'd recommend an A5 sized hardback or ringbound artists sketching pad. If you get a good quality one with a page weight of around 150 gsm, this will take light washes of watercolour. The A5 sizes are quite convenient as they can be carried in a small bag or large inside pocket. If you want to use heavier paint such as acrylics, a weight of around 250 gsm would be better although these usually come in A4 size. To stop your pages blowing about, just use elastic bands around them or bulldog clips.
When you get home I often find it better to transfer the sketches/paintings to a D ring binder and either punch holes in the paper or use document sleeves, although this applies more to the A4 ringbound types. You can then group them in categories such as birds, insects etc. You can also include photographs/printouts as well.
Of course as well as notes, drawings and small field paintings, use a camera too. Small digital ones are quite adequate for quick record shots; just make sure it has a macro facility.
Also Dictaphones/small tape recorders have there uses and PDA’s/notebook PC’s are getting more common.
I've included a few images of the ones I use. But beware; you could end up with a large collection of sketches, field paintings, photos, notes etc
