Behaviour of female blackbirds

Observation - Behaviour of female blackbirds - UK and Ireland. Description: No photo, just a question about behaviour of female blackbirds. Our garden is frequently visited by two female blackbirds, one of which chases the other around and around the lawn and everywhere it goes - on the fence, in the trees, etc. They always lea

No photo, just a question about behaviour of female blackbirds. Our garden is frequently visited by two female blackbirds, one of which chases the other around and around the lawn and everywhere it goes - on the fence, in the trees, etc. They always leave together, one closely chasing the other. Any ideas to explain this behaviour?