Gorse is a nitrogen-fixing legume which can invade heathlands and grasslands. It regrows after burning or cutting. Its young shoots may be grazed, but mature branches are too spiny. Gorse provides a nesting site for birds such as the yellowhammer and whitethroat. Broom is another invasive woody N-fixing legume. Bracken is one of the commonest and most widespread ferns in the world. It is often the dominant vegetation on heaths, moors, open woods and neglected pastures, especially on acid light soils. Burning encourages its spread since this destroys other plants but does not harm the deep rhizomes of bracken.