This is a shame because there really is a place for women-only bird-watching walks. The reason is based in male/female socialisation.
Men can be terrible know-it-alls in mixed groups and interests like bird-watching, that attract people with a collector mentality, who may lack well-developed warm, inclusive social skills and can view anything involving lists as competitive, can be natural homes for the worst sort of noisy, know-it-all behaviour.
A walk for women, where all the people talking are women, would feel very, very different and would provide an opportunity for learning and enjoyment that doesn’t otherwise exist.
For that reason the relevant group for the activity is the sex class of women as female people who were brought up as girls.
Unfortunately the tendency towards collector and competitive mentality goes hand-in-hand with a rights-based perspective, not with a warm, empathetic, insightful perspective on DEI - and this mentality will be normal amongst their staff.
Lack of empathy means rights tend to be looked at from the perspective of the thinker - or the nearest example they can muster of ‘what might this be like for me’. If ‘me’ is a confident, competitive, rules and rights-based birdwatcher, of either sex, that doesn’t get them very far.