"Overall, we project that spring plant phenology will shift by approximately three weeks earlier in the year across the continental United States by the year 2100, with considerable regional variation," Andy Allstadt of the University of Wisconsin-Madison, US, told environmentalresearchweb. "For example, the Pacific Northwest and mountainous regions had the largest changes, while southern locations often had little change, because spring already arrives so early there."

Allstadt and colleagues used the RCP8.5 and RCP4.5 scenarios to make daily temperature projections with the Coupled Model Intercomparison Project 5 ensemble. They fed the results into the extended Spring indices, originally based on observations of lilac and two types of honeysuckle, to calculate dates for leaf out and first bloom from 1950–2100.

The researchers have a particular interest in how bird populations across the United States respond to changes in weather and climate. They reckon changes in the timing of leaf out or flower emergence can affect birds in two ways.

"First, long-distance migrant birds are tired and hungry, and need to rest and refuel along the way, so they arrive on their summer breeding grounds ready to establish territory, find a mate, and begin breeding," said Allstadt. "Their migration and arrival on the breeding grounds is often timed to match the availability of plants and insects to support their activities. But plant growth and insect emergence on the breeding grounds is determined by the weather at those locations, and birds don’t get the weather channel."

Instead, the birds time their migration by changes in day length on their wintering grounds and fine-tune their movements using weather pressure systems, Allstadt says. "However, day length is not changing, so if spring arrives earlier and earlier, birds who don't begin migrating until the day length is just right may miss out on important peaks in food."

In Europe, studies have found that some species of bird are migrating earlier, whereas others aren’t. "We may find that the early bird indeed catches the worm and the ones that don't or cannot change may see their populations decline," said Allstadt.

An increase in false springs could also affect birds. If there’s a hard freeze – with a temperature less than –2.2°C – after spring plant growth has begun, it will damage the plant, reducing its productivity and reducing food for animals. "If flowers are already out, they tend to be particularly sensitive to the freezing temperatures, meaning fewer fruits and seeds later in the year," said Allstadt. "For humans, this is important because the false spring can damage agricultural systems. In 2012, a widespread false spring in the United States caused an estimated damage of $500 million, just in the state of Michigan."

False springs look set to become more likely in the Great Plains and parts of the Midwest as climate changes to 2100, but remain about as frequent or become less of a risk elsewhere, the researchers found.

The team is working on a NASA biodiversity project to determine how bird populations across the US respond to changes in weather from year to year. "Do they move elsewhere? Do they not reproduce?" said Allstadt. The researchers have created datasets measuring droughts, heat waves and other extreme weather, and are examining the historical effects of weather on birds, as well as projecting these changes into the future.

"Another part of our project is to share relevant weather data and climate projections directly with conservation managers," said Allstadt. "As part of that, we are making all of our weather data, including the spring phenology and false springs data, available through our lab website."

Allstadt and colleagues reported their work in Environmental Research Letters (ERL).

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