False Springs: How Earlier Spring With Climate Change Wreaks Havoc on Birds

The last few decades have seen an advancing onset of spring as temperatures warm due to climate change. Early spring may seem like something to revel in, but for natural ecosystems it can wreak havoc. Given the rapid pace of climate change, long established relationships can unravel, leading to mismatches in species phenology—the timing of seasonal and cyclic component of a species life. 

This phenomenon can be seen with bird migration: millions of birds are migrating earlier as temperatures warm. Some species like the Black-throated Blue Warbler are migrating nearly a week earlier than they did 50 years ago. However, not all species are able to adapt as rapidly as others, subsequently leaving them a step behind during the integral breeding time. This can be as simple as birds like the Broad-tailed Hummingbird missing out on a key food resource needed for breeding, or as bizarre as birds turning into zombie-like killers to defend limited resources. 

Although birds a migrating earlier in spring in general, a recent study showed that across all bird species groups migratory behaviors are not adapting fast enough to keep pace with a rapidly changing climate. As warmer temperatures earlier in the season cue a green up of new plant growth and lead to an earlier peak in food resources for birds locally at the breeding grounds, bird species are failing to show up in time. This is especially pronounced in long-distance migrants like the Cape May Warbler that travel to Central and South America during the non-breeding period and are relying on hardwired behaviors, such as perception of day length, to trigger migration.  

Early onset spring is especially problematic if freezing temperatures occur after plants have started to grow. These ‘false springs,’ where sustained warm temperatures in late winter or early spring cue vegetation growth and then is followed by an extreme cold snap, can cause vegetation damage and cascading ecosystem effects. These types of extreme events are well known to fruit farmers, who can lose whole crops and face millions of dollars of damages due to the effects of a freeze on tender young buds.

False springs can be devastating to birds and other wildlife. Birds may be directly killed by unseasonably cold temperatures, which can be exacerbated if cold snaps are accompanied by freezing rain. Losses are most pronounced in migrants like Scarlet Tanagers that overwinter in the tropics and are not adapted to colder conditions. Ice storms during this time have been known to encase migrating Common Loons in ice, causing them to fall from the sky, become stranded on dry land and die if not rescued. 

Cold snaps and storms associated with false springs also lead to decreased food availability. Insectivores in general are prone to mortality as their insect prey go dormant in cold weather. This has been seen in Cliff Swallows where in some cases more than half of a population died over a single 6-day event. Water foraging birds are also at risk if temperatures are cold enough long enough to re-freeze ice, locking food below and out of reach. If snow or ice are accompanied with colder temperatures, ground foraging birds may also be at a loss for finding food needed to replenish their fat stores after migration. 

Reduced availability of key resources needed to rear young during cold snaps also has implications for breeding success, where complete breeding failure has been observed in shorebirds like the Red Knot. Tree Swallows are breeding earlier in response to advancing springs, but now experience high rates of chick mortality from double the amount of risk from inclement weather and associated reduction in insect food availability. Indeed, breeding attempts during a cold snap have higher rates of nest failure across many common and widespread species. Although spring has advanced considerably, the average date of the last cold snap has not changed, indicating that bird species now face the added challenge of facing a longer window of potential climate related hazard during their breeding season. 

All of this considered, early spring doesn’t seem so attractive if you are a migratory bird.

Abrupt changes in weather can also pose a risk to birds migrating south in the fall. Although not a false spring, the implications of unseasonably cold weather during fall migration are similar. The  deaths of migratory birds in the southwest may have been caused by a rapid change in weather conditions. As the migratory season is getting longer overall in fall and late fall migration extending later in the year, the likelihood of migrating birds encountering cold snaps on their way back south may increase.

With climate change, spring is expected to continue to shift earlier and false springs are expected to become an increased risk for portions of the U.S. By the end of the century, winter may last only two months, starting later and ending in an earlier transition to a colder and earlier spring season with more risk of false springs. Although temperatures will continue to rise, cold snaps and associated storm conditions are likely to persist during key migration periods for birds. 

What can we do to help migratory birds cope with the dramatic effects of early springs? Taking steps towards stabilizing the climate by reducing carbon pollution and conserving and maintaining more landscapes will make the biggest difference. There are also everyday actions people can take in their homes and communities: 

• Growing native plants will provide natural cover and food sources for birds. Adding bird feeders and water sources to your yard can also provide supplementary sources in times of need. 
• Turning off or dimming non-essential lights at night and making reflective glass more noticeable will give birds a safer passage during migration.
• Becoming a community scientist and submitting your bird observations to eBird provides a great record of migrating timing and helps us understand how things are changing for birds with climate change.