Birds finding it hard to adjust to shifting springtime

But first, a new study suggests that the times at which plants start to display their green shoots are shifting - but bird migration in the spring is not adjusting accordingly. So, what’s going on, and what might the consequences be? I’ve been speaking to Oklahoma State University’s Ellen Robertson and Scott Loss…

Scott - The climate is changing worldwide and one of the big things happening is the changing of the seasons. One of the patterns that we're seeing is the earlier arrival of spring and with that earlier leaf out and flowering out of trees and the associated food resources that accompany that. Our question was to really delve into the degree to which migratory birds are tracking or not tracking that change in the timing of spring. There's a lot of concern about are birds gonna be able to time their migrations to track the changing timing of spring or are they gonna increasingly be disconnected from that with potential implications for their populations and persistence?

Chris - And how did you get at that question Ellen?

Ellen - We were able to use a gigantic citizen science database from eBird that was collected by volunteer birders from all across the western hemisphere over a 20 year period. We connected that to data from satellites up in space that are collecting data on vegetation greenness on the ground to understand how climate change is affecting bird migration.

Chris - So presumably the bird watcher data gives you a time and a place and a species when different things arrive in different places and when. And the satellite data tells you what nature's doing in the geographies that these animals are coming to. So you can look at whether there is joined up migration or disconnect beginning to emerge.

Ellen - Right, exactly. And we did this for 150 bird species and so this just shows the power of community science and all of these people collecting this data combined with this enormous satellite data that's being collected from space.

Chris - Scott, can you give us a sort of high level overview as it were of what emerged from this? So first of all, what did nature do differently across the 20 years of the study in terms of the arrival times of spring? Was there a genuine trend there that shows spring coming earlier?

Scott - So there's variation from year to year in terms of the arrival of spring, but really at a large scale and across the 20 years of data that Ellen looked at, there's still a consistent signal for spring happening earlier.

Chris - When you say it's coming earlier, how much earlier? Are we talking a few days adrift or is it things are beginning to move maybe a month or so, something significant like that?

Ellen - There is a lot of variation but it looks like from the maps of this year so far, a lot of the United States is 10 to 15 days earlier than long term averages.

Chris - And what about the birds? What do they do?

Scott - One of the major questions with this research was the degree to which migrating birds in spring can track the changing green-up pattern that we're seeing with spring coming earlier. And this is the first study to really address that specific question across numerous bird species and 110 or so of the 150 species are more closely tied into the long-term averages of greenup, which suggests that their migrations are more static and consistent towards past springs, not the current springs that they're experiencing. And there's also evidence that that earlier spring green-up is increasing the gap or contributing to an increasing gap between the timing of green-up and the timing of bird migration, especially birds that migrate very long distances, uh, wintering in places like the Caribbean, central and South America and come to the us, Canada, et cetera, to spend the summer. Those species are the least flexible it looks like.

Chris - I suppose the problem for them, Ellen, is that when they depart from wherever they've spent the winter, they've no idea what the weather is doing and therefore what spring is doing in their target destination till they get there. And by the time they do, sometimes it's too late.

Ellen - Exactly. That's thought to be a problem of migration. Very complicated. Birds use magnetic fields, stars, the sun, different landforms on the ground in terms of the long distance migrants. There's some evidence that they're more triggered by things like day lengths and things that don't vary and so they may be less flexible as conditions are changing along their migration roots

Chris - Is the reason this is such a problem for these migrating species that if they arrive and they want to have babies, then spring is already sprung and various insects and things that they would rely on to feed their babies, those presumably have like a caterpillar, have now become butterflies and they're no longer a potential food source. Is that the problem?

Ellen - Yes, that definitely is a huge issue. And there is some evidence that there are these demographic mismatches occurring with birds when they're breeding, that it is having some consequences for their fecundity when either spring is very early relative to normal or very late, they're not as matched and so they're not reproducing as well.

Chris - And so Scott, is there evidence this is being reflected in declining populations of birds, for example, are the birds clearly being impacted by this or do they appear to be able to tolerate it, albeit that the conditions are less ideal for them?

Scott - That is the big question, what do these, the increasing gaps between the timing of migration and the timing of spring and spring green op, mean for bird population? Persistence? There are suggestions that if you are missing your key resources during migration, there could be some implications to you during migration. Your physiological condition and health during migration might be affected. That can affect your ability to complete migration, let alone make it to the breeding ground and reproduce. Birds are in decline worldwide. There was a well-known paper in the US a few years back ago showing that we've lost about a third of our birds over the last 50 or so years. And there's a whole host of reasons for that, like habitat loss, chemical use, um, invasive species, et cetera. Climate change and associated disturbances with that has to be a part of it as well. But figuring how these types of climate change related disturbances play into those declines is an open area of inquiry.