Spring officially started this week, but for many of us it had already begun. Back in February, unseasonably warm temperatures swept over much of North America, buds began opening on trees, and flowers began to bloom weeks early. Naturally, birders began to ask: Will our migratory birds come back earlier, too? That question doesn’t have a simple yes or no answer because the timing of bird migration is . . . complicated. Every species is slightly different; short-term changes in weather do have an impact, but so do a variety of other innate and environmental factors. Here’s a quick primer on how North American avians schedule their spring journeys to aid in your own birding ventures. Two Types of Migrants To figure out how migrating birds could be affected by balmy weather, we should start by categorizing them into two groups: obligate and facultative (to use the fancy terms). These labels aren’t ironclad—many birds fall somewhere between these extremes—but the...