Familiar signs of spring are gathering in the garden. Bird droppings move miraculously along the leaves of our lemon trees, while butterfl ies with handbreadth wingspans fl utter languidly overhead. The swallowtails are early this year, their breeding cycle hastened by an unusually warm winter. Our orchard swallowtails usually emerge from their winter cocoons in late September. This year, however, they have already laid their fi rst batch of eggs, and the early instars are chomping into our citrus leaves, ignored by hungry birds unwilling to tuck into what appears to be fresh dung.Over the next fi ve weeks, the instars (caterpillars) will moult and grow, turning greener and sporting two bright red horns that spring out when the caterpillar is harassed, spraying a pungent, irritating chemical over the unfortunate predator.With a wingspan of 13cm, this is one of our largest butterfl ies, and its brief adult life is devoted to dispersal and reproduction. Males often have to search long and hard to fi nd females to mate with, guided by wing patterns that are hidden from our eyes. Each wing bears more than one million overlapping scales. Some are pigmented, while others are transparent and split the light into brilliant iridescent patterns in the ultraviolet and infra-red spectrum.When a male fi nds a female, he shows his readiness to mate by hovering close to her and fl uttering his wings. If she is receptive, copulation may last for an hour or longer.The female lays her eggs on the leaves of plants that provide food for the caterpillars. These include native orange and fi nger lime as well as introduced citrus. Planting citrus trees will encourage these beautiful butterfl ies to visit your garden and help to maintain this species in urban areas.The early breeding of our swallowtails is a sign that climate change is infl uencing the life cycles of plants and animals. Scientists are monitoring these processes, and are calling on the public to help by recording sightings of swallowtails and other species using the free ClimateWatch app.