(Above: Migratory songbirds in the HSA, like the prothonotary warbler, rely on insects in the spring during breeding and for fledglings. Photo Credit: Doug Tallamy)
This is the second article in a series about climate change impacts on various ecosystems. To read the other articles in this series, click here.
Here we are folks, spectating one of the great races in our natural world. A race against time. A race against temperature. A race where there are only winners and losers, no first, second, or third. This is a race for survival.
In the left lane is the high-profile racer, Migratory Birds. Against them is the ‘under-the-radar’ competitor: Insects. Migratory Birds and Insects are familiar opponents, having competed against one another many times before. It the past races Migratory Birds won, but scouts suggest Insects have altered their strategy and are gearing up for a win. This race is an interesting sport to watch unfold; let’s see how it goes.
I’m sure we all look forward to the spring migration. It is one of our regions most exciting times to be a naturalist. All the wonderfully coloured birds come back to us in droves, with their songs and sounds to uplift us after the winter. Migration, though, is a risk that birds are seemingly begging to suffer from in recent years. Migratory birds are racing against each other to stake out the best territory and mates, but also, increasingly, against the breeding ground itself. The most important food sources for arriving and breeding birds are insects, which overwinter on the breeding grounds.
Fat- and protein-rich insects are an important food source for songbirds in the spring, when energy demand is highest for migration recovery, nesting, and chick-raising. When insect abundance is low, songbirds have a less delectable spring buffet, causing some chicks to make do with little. Unfortunately, in nature there are no food bank programs. In what would be considered a ‘typical’ year, songbird arrival and breeding occur during peak insect abundance, usually caterpillars and large flying insects, to much benefit of the birds and hardship of the insects. Though climate change is changing this relationship.
As winter and spring temperatures continue to warm, the insects are developing sooner than songbird arrival. Insect growth and development is largely controlled by temperature. As temperature warms, insects grow faster. Increasing temperatures in the northern hemisphere, especially at higher latitudes, is causing peak insect abundance to occur earlier in the year than in the past. A study conducted in 2017 used satellite imagery to measure changes in spring vegetation growth (an indirect measure of insect growth) over 12 years across North America. The researchers found that vegetation was developing at a rate of 0.952 days earlier every year, or 9.5 days earlier every decade (Mayor et al., 2017). Migratory birds must adjust their arrival accordingly to maintain their food rations.
Yet, a main migration signal to birds is daylight hours in their wintering grounds. Daylight hours remains the same year after year, but temperature is changing. Albeit, there are many different migration cues, the ability to predict when to start moving is difficult when you are a long way from home (if we can call here their home) or even on a different continent! In the same 2017 study, average bird arrival across North America changed at a rate of 0.668 days earlier per year, or 6.7 days per decade (Mayor et al., 2017). Across North America, it is the eastern forests birds that seem to be changing quickest, though still not fast enough. What seems to be occurring is that birds are continually arriving farther and farther behind peak insect abundance, missing out on the full array of optimal food on which the birds once relied. There may be long-term impacts on the ecosystem with high insect abundances ungoverned by bird predation.
The long-term impact on the entire ecosystem is still rather unknown. From what we have seen so far through, are three possible outcomes: 1) migratory birds will continue to lose ground and drop out of the race, 2) the birds are saving for a last 100m dash, or 3) the birds will alter their strategy for the long-term success.
SCENARIO 1: the long-term impacts are likely to occur in the ecosystem is defoliation of select species- mostly oaks, beeches, ashes, and birches; and the dominance of the remaining hardier species like maples and pines. Tent caterpillars, for example, prefer the softer and more digestible aspens and birches and tend to avoid bitter and fibrous maples. In young forests, this may have dire consequences as aspens and birches are some of the first trees to start growing, kickstarting forest regrowth. Other spring caterpillar species will impact their host plants likely in similar ways. Non-migratory birds may increase in abundance to fill the roles being left open by the disappearing migratory birds. This is the only scenario where the insects outrightly win the race and the birds lose. Long-distance migratory birds like the Scarlet Tanager are the most likely to lose the race.
WINNER: Insects. LOSER: Migratory Birds.
SCENARIO 2: there are works that suggest that individual birds are impacted, but the population is not (Visser, et al., 2006; Alves, et al., 2019). Natural selection is thought to take over in the long run, allowing for those earliest birds to become the strongest and most successful, thereby winning the race on their own terms. This will have no long-term impact on the ecosystem other than perhaps an increase in insect abundance during the transition period. The birds lose the relay rounds, but not the entire race.
WINNER: Migratory Birds. LOSER: Insects.
SCENARIO 3: using the Barn Swallow as an example, concerns over the current declining numbers pointed towards fledglings not getting enough good quality insects in their diets in the spring. Since they are migratory, insect-eating birds, it was a reasonable explanation. A group of studies out of Trent University in Peterborough tested this idea, it was found that although the fledglings were hatching at a time after previous food species peaked, there were seemingly no impacts on the fledgling growth (McClenaghan, 2018). The parent swallows instead changed their food preference to a more abundant food species available at that time. In another case, Black-throated Blue Warblers have also been recorded as arriving after peak food abundance, but instead of changing their diet, they have shortened the time between nest building and egg laying as an attempt to stay in the race long enough to get a foot-hold (Lany et al., 2016). Other bird species have changed their breeding timing in various ways, with various successes- some are not successful (McDermott & DeGroote, 2016).
WINNER: Insects and Migratory Birds (Eastern Phoebe, Red-eyed Vireo, Gray Catbird, Indigo Bunting);
LOSER: Insects and Migratory Birds(Cedar Waxwing, Ovenbird, Hooded Warbler, American Redstart).
This race between birds and bugs will trigger some cascading ecological impacts due to the force of climate change. The forests in Ontario will likely look different over the next 50-100 years. For us birders, our year lists may get smaller as a result. It will be interesting to see the racers change pace and strategies over the track.
For further reading:
Alves, J.A., Gunnarsson, T.G., Sutherland, W.J., Potts, P.M., Gill, J.A. (2019). Linking warming effects on phenology, demography, and range expansion in a migratory bird population. Ecology and Evolution, 9, 2365-2375. doi: 10.1002/ece3.4746
Lany, N.K., Ayres, M.P., Stange, E.E., Sillett, S., Rodenhouse, N.L., Holmes, R.T. (2016). Breeding timed to maximize reproductive success for a migratory songbird: the importance of phenological asynchrony. Oikos, 125, 656-666. doi: 10.1111/oik.02412
Mayor, S.J., Guralnick. R.P., Tingley, M.W., Otegui, J., Withey, J.C., Elmendorf, S.C., Schneider, D.C. (2017). Increasing phenological asynchrony between spring green-up and arrival of migratory birds. Nature Scientific Reports, 7, 1902. doi: 10.1038/s41598-017-02045-z
McClenaghan, B. (2016). Using DNA barcoding to investigate the diet and food supply of a declining aerial insectivore, the Barn Swallow (Hirundo rustica). Trent University, Environmental and Life Sciences Graduate Program. [Thesis].
McDermott, M.E., DeGroote, L.W. (2016). Long-term climate impacts on breeding bird phenology in Pennsylvania, USA. Global Change Biology, 22, 3304-3319. doi: 10.1111/gcb.13363
Visser, M.E., Holleman, L.J.M., Gienapp, P. (2006). Shifts in caterpillar biomass phenology dye to climate change and its impact on the breeding biology of an insectivorous bird. Global Change Ecology, 147, 164-172. doi: 10.1007/s00442-005-0299-6