This poem takes as its starting point the experience of hearing birds singing in the woods on a May morning. The cockthrush is claimed to be a love-messenger sent by Morfudd, and its song is likened both to a poet's englyn and to the words of the mass. This is then developed into an extended metaphor of the woodland mass, with the thrush as priest, flowers as its vestments, a leave the communion wafer, and the nightingale as the sacrist assisting the priest. The imagery contains implicit criticism of the human priest muttering and mispronouncing the Latin words in the parish church. But there is a potentially blasphemous climax to the imagery, since the communion chalice contains nwyf a chariad ('desire and love').

The metaphor of birds as priests is common enough in medieval European literature, and this poem could have been influenced by a French poem of the early fourtennth century by Jean de Condé, La Messe des oiseaux (see Jacques Ribard (ed.), La Messe des oiseaux et le Dit des Jacobins et des Fremeneurs (Geneva, 1970), APDG 76–7, and DGIA 233–5). But there is no detailed correspondence between the two poems, and it is more likely that Dafydd developed the imagery independently of any specific literary influence.