Complex sentence: Birds, especially waders, have blood-vessels in their lower legs which act as heat exchangers-veins are right next to arteries and thus extract heat from the arteries and carry it back into the trunk.
Split into simpler sentences: Birds, especially waders, have blood-vessels in their lower legs which act as heat exchangers. The veins are right next to the arteries and thus extract heat from the arteries and carry it back into the trunk.