When the sun begins to warm up the northern hemisphere again in spring, they head north once more for a new breeding season.
(the sun; begins; ) (the sun; to warm up; the northern hemisphere; T:again; T:in spring) (they; head once more; T:When the sun begins to warm up the northern hemisphere again in spring)
the sun the sun they
begins to warm up head once more
the northern hemisphere T:again T:in spring
T:When the sun begins to warm up the northern hemisphere again in spring