House Martins collecting weeds (1 Viewer)

Recently (end of May 2019) I saw House Martins nesting on buildings near Canary Wharf in London collect weeds from the muddy bank of the Thames at low tide, carrying them to their nests (see photos). What would they need the weeds for? I thought that the nests were made from mud only, and that their diet includes only insects. Do they use the weeds to line the nests?

I think they also collect dried grass and feathers to line the nest, occasionally using the plant material to bind the mud pellets, a bit like "lathe and plaster". Swallows do this as well.

Thanks.