The Yellowhammer is a robust 15.5–17 cm long bird, with a thick seed-eater's bill. The male has a bright yellow head, yellow underparts, and a heavily streaked brown back. The female is much duller, and more streaked below. (Wikipedia)
It is most commonly found on lowland arable and mixed farmland, probably due to the greater availability of seeds. It nests in hedges, patches of scrub, and ditches, especially if these have a wide grass margin next to them, and a cereal crop next to the margin. Hedges of up to two metres tall are preferred, and they will not nest until it is in full leaf, building the nest next to the hedge if it is built before this. In winter, the flocks feed at good seed sites, such as newly-sown fields and over-wintered stubbles. (Wikipedia)
These birds are coming only winter to our garden, I guess because they don't have seeds on fields to eat.