In this task, you're given an article and an answer. Your task is to generate the question for the answer based on the given article.
Q: Article: Starving polar bears are eating one another in the Arctic. Flowers are blooming too soon and die. The ice caps are melting so fast that rising water levels will threaten coastal towns along Florida within several decades. These are just a few examples of the terrible consequences of climate change supported by a new analysis inNature. In the past three decades, average global temperatures have risen about 0. 6degC and are _ to jump by about 1. 7degC by the end of the century, says Cynthia Rosenzweig, who leads the Climate Impacts Group at NASA's Goddard Institute for Space Studies at Columbia University in New York. "We've already seen that a relatively low amount of warming," she says, "can lead to a broad range of changes. " The unnatural warming caused by man-made greenhouse gases, especially carbon dioxide produced by cars and coal-powered plants, brings trouble for entire ecosystems. In North America alone, scientists have identified 89 species of plants, such as the American holly that have flowered earlier in the spring. In Spain, apple trees bloom 35 days ahead of schedule in response to the higher temperatures. Other wildlife, like the insects that use certain plants for food and the birds that feed on the insects, must then move forward their seasonal stirrings and mating patterns to survive. To try to follow this time shift, some birds such as robins, the classic symbol of winter'sthaw , are returning to Colorado from their migrations some two weeks earlier than in years past. All these changes can throw a food chain in disorder. Some bird species that arrive before the insects reappear may starve to death. "Around the world, plants and animals are waking up to an earlier alarm clock than they used to," says Terry Root, a biologist from Stanford University.
Answer: They used to come back when spring came.
A:
What can we know about robins according to the passage?