With climate change causing spring to start earlier each year, and with migratory birds reliant on timing their arrivals with available food items and nesting sites, researchers have now used millions of bird sightings from ordinary birders to evaluate these changes.
The precise beginning of spring differs from year to year and biologists have wondered how well birds were able to track this change, especially given consensus forecasts for a warmer and more variable climate. Migration is linked to weather from year to year, but birds are also hungry after long migratory flights and time their arrival for when the trees begin to produce leaves and caterpillars emerge to feed on them, providing an important source of protein that contributes to birds' survival and reproduction.
Arriving at the optimal time is vital, because birds must be late enough to avoid cold conditions but early enough to catch the spring pulse in food and establish nest sites and territories. Researchers investigated 48 North American songbird species, which were found to be falling out of sync with the timing of the spring 'greening' by an average of five days per decade. More worryingly, some species were becoming increasingly mismatched to their environments by two or three times that rate.
Northern Parulas are arriving earlier each spring, but are not keeping pace with climatic changes (William H Majoros (commons.wikimedia.org)).
Cornell University's eBird program is used to connect birders' field sightings with research and conservation to provide a massive database which will ultimately indicate overall trends in every species recorded. The data can be entered for any site in the world, and checklists logged by North American birders have been used in more than 100 peer-reviewed papers, as well as hundreds of local, regional and national conservation decisions. One of the most recent of these papers is 'Increasing phenological asynchrony between spring green-up and arrival of migratory birds' in the journal Scientific Reports which has potentially bad news regarding the changing climate.
The team found markedly different patterns in eastern and western forests. In the eastern temperate forests from southern Canada to Florida, the spring greening generally advanced. Birds such as Northern Parula and Yellow-billed Cuckoo also arrived earlier typically, but did not keep pace with the change of spring. In the western forests, spring greening unexpectedly became later over the period between 2001 and 2012, and birds like Townsend's Warbler also arrived later. Just as in the east, they adjusted their arrival times in the right direction but didn’t keep pace with climatic and ecological changes.
The increasing phenological mismatch may contribute to bird population declines, something the researchers will be further investigating. It is also not clear why some species seem to be growing out of sync with their environments while others seem to be doing just fine.
The study is available for free in Scientific Reports here.