Every year, the beginning of spring seems to inch further and further up the calendar year, like a caterpillar inching up a branch. The official start date of spring for humans is March 20th, but the seasons for wildlife operate on a different schedule that is cued in large part by temperature and light. Flora and fauna use those cues to determine when to blossom, when to migrate, when to die. There is a branch of scientific study focused on the relationship between the life cycles of plants and animals and climate. Phenology is the study of how climatic conditions influence the timing of natural phenomena, such as trees losing their leaves, caterpillars transforming into butterflies, and birds migrating south for the winter. The climate is also intricately linked to the return of birds to their nesting grounds, but not in the way you would think.
Leaves now tend to unfold from the buds on trees and attract insects in large numbers before the birds have swept in on the warm spring air. Some bird species are arriving later than spring leaf-out (when leaf buds on trees open), and are therefore missing out on the great bounty of insects chewing on the plants. Other changes are taking place in the bird world. Some bird species are expanding their territories north, and some species are staying through the winter where they haven’t before. One would think that birds can intuit that spring has arrived in their nesting grounds, and migrate back and forth like clockwork, but can they?
To gain a better understanding of the correlation between migration patterns and climate, it is helpful to look to studies that use big data sets. In “Birds of Hamilton and Surrounding Areas”, Burlingtonian Robert Curry uses 150 years’ worth of local data to show that there have indeed been changes to the average spring return dates of some birds in the Hamilton area. Curry compares data from two time periods,1950-1970 and 1980-2000. One pattern that emerges is that the ranges of many bird species, such as Turkey Vultures, Mourning Doves, Red-bellied Woodpeckers, Northern Mockingbirds, Hooded Warblers, and Northern Cardinals, are moving further north into southern Ontario, and rising temperatures are likely a contributing factor. Not only are some bird species now making southern Ontario their permanent home, there are also emerging patterns in migration.
Temperature is what signals birds to fly south, but what signals them to fly north again, when their winter home is in a tropical climate that has a consistent temperature? The answer to this question becomes more apparent when we compare the different migration times of various bird species. For Hamilton and its surrounding areas, Curry classified twenty species in the data set into early, mid-spring, and late-spring migrants based on their arrival dates. Most of the early migrants he lists only migrate as far as 1000 kilometres away from Hamilton, winter in the Southern USA, and arrive back in our area in early spring. Mid-spring migrants travel further than the early-spring migrants, and arrive in mid-April. Most of the late-arriving species are long-distance migrants that winter the furthest away in the West Indies, Central and South America.
Curry notes that the average first arrival date of a number of species have changed; however, when one looks at the chart of twenty species studied, the asynchrony is not uniform across all species. Early spring, mid-spring, and late spring migrants have responded in different ways. The arrival dates of early-spring migrants have changed the most. Instead of quoting in exhaustive verbatim from Curry, I’ve selected a few species as representatives of early, mid, and late spring migrants to illustrate the results. Turkey Vultures, for example, are early-spring migrants and boast the most significant change in migration timing. From 1950-1970, on average, Turkey Vultures arrived on March 27th. In 1980-2000, Turkey Vultures then arrived, on average, on March 6th: that is a substantial difference of twenty-one days. Another early-spring migrant, Tree Swallows, went from arriving on April 3rd between 1950-70, to March 23rd between 1980-2000: a difference of eleven days. Turkey Vultures appear to be keeping up better with the earlier arrival of spring weather. In contrast with that species, the average arrival date of Barn Swallows, a mid-spring migrant, was only earlier by five days. An armchair scientist could surmise that mid-spring migrants are becoming further out of sync with the first date of spring. The most disturbing changes were the ones that didn’t happen at all as seen in the arrival dates of late-spring migrants who tend to travel the furthest. Late spring migrants, like the Ruby-throated Hummingbird, still arrived, on average, on the same day, May 7th, in both time sets. While these results are not completely conclusive nor representative of all migratory birds, the difference of average arrival dates in contrast to the first day of spring should still raise worried eyebrows.
A more recent study using eBird data found that at least forty-eight songbird species have fallen out of sync with spring by five days. Another study, by Deleon, revealed patterns similar to those outlined by Curry. In the Deleon study, 37 early spring (short-distance migrants) moved their arrival dates ahead by .15 days a year, arriving on average as a group nearly a week earlier in 2008 than in 1967. In contrast, late spring (long-distance migrants) only moved their dates ahead by .06 days a year changing only 2 and a half days from 1967. Back in Halton, Turkey Vultures are observed in numbers now even in January and February. Birds that were previously rare, or not known to stay through the winter (for example Ruddy Ducks and Eastern Bluebirds), now stay through the winter regularly.
What is happening in the environment that so many bird species migrate home at increasingly asynchronous dates with spring? According to Environment and Climate Change Canada, and Statistics Canada, the average temperature all over the world has gone up 0.85 degrees Celsius from 1880 to 2012. A global average doesn’t reflect the significant local increases in temperatures across the world. While temperatures in the tropics haven’t greatly changed, there is a measureable change towards warmer temperatures in the temperate zone where we live. In fact, Environment and Climate Change Canada have stated that ‘the rate of warming in Canada as a whole is more than double that of the global [rate]”. Over the last twenty-five years, average winter temperatures have gone up for overall milder winters, and because winters are milder, we’ve seen less snowfall. Tropical air coming over the oceans arrives earlier each year in March or even February. In Eastern temperate forests, spring arrives early, and in the west, spring arrives late. The extreme temperatures have changed across the country, and there is a higher frequency of hot days and heat waves, and fewer cold days in the winter.
Temperature is important because it provides cues, along with sunlight and precipitation, for flora and fauna to begin and end life cycles. When the weather changes from warm weather to cool weather and days become shorter, plants use those cues to begin to drop leaves during autumn. Conversely, the change of cold to warm weather spurs plants to put out leaves. Leaf-out is another cue in nature: when leaves and fruit appear, so do swarms of insects to eat them. Ideally, migrating birds take advantage of feeding opportunities on route and arrive home in time to rear young during the peak period of insect abundance. Students learn about trophic levels and food chains, also known as the food web, in school. Birds eat insects, insects eat plants, and plants grow in warm weather. The problem is that this web of life is at risk of major disruption in a changing climate. Because birds aren’t as sensitive to changing temperatures, they are at risk of being, or are already, out of sync with food availability on their breeding grounds.
Some species of migratory birds are more in sync with the bounty of insects and leaf-out than others. Short-distance migrants tend to be responding faster to warming temperatures than many long-distance migrants. There is a possibility that since the birds don’t travel out of the temperate zone, the local temperature of their wintering grounds is rising at the same rate as their breeding grounds up north. The weather warming up in their wintering grounds is a cue for them to migrate. The difference in local temperature could explain why late spring, long- distance migrants are becoming more and more of out of sync. The tropics haven’t been affected by rising temperatures as much as the temperate zone has. Climate change is more pronounced in temperate zones than in the tropics. Warming temperatures in the US may accurately signal short-distance migrants on the best time to leave for breeding grounds. The most important part to understand, when detangling the patterns of phenology, is to understand the link between temperature, migration, and food.
Temperature can cue migration southwards, but not necessarily northwards. Birds wintering in the tropics wouldn’t use temperature as a cue to migrate home in time for the plentiful feast of insects. Purple Martins, for example, are long-distance migrants that arrive in late spring. They are becoming further out of sync with leaf-out and spring with each passing year. Purple Martins winter in South America, and nest in Baja California, Mexico, and Canada. These birds always arrive on the same few dates. In 2012, York University ran a study using geolocators attached to about fifty Purple Martins. Temperatures in the Amazon basin where the Purple Martins overwintered were normal, while the US experienced the warmest spring on record. The Purple Martins left the Amazon basin at the same time that they do every year, and they arrived much later than the peak period for insects. Researchers hypothesized that maybe the Purple Martins might use other environmental cues to fly home, or would sense the temperature changes as they migrated, and accelerate to catch up. There was no evidence that the Purple Martins accelerated or that they were physically capable of accelerating. The scientists involved in the study surmised that natural selection could eventually address the problem of the Purple Martins falling further and further out of sync, but natural selection is a slow, unpredictable process. It is more likely that the Purple Martins will continually fall out of sync with leaf-out, and eventually die out, as they’re left with scraps to eat—unable to adapt in time. We could lose bird species not because they could die in the cold, but because there is not enough food for them to eat and successfully raise young when they arrive home.
While some species are falling further out of sync with leaf-out, other species are setting up home further north and no longer migrating south at all. Eastern Bluebirds are choosing more and more to overwinter instead of migrating south–a phenomenon observed at the Conservation Halton Administrative Office where bluebirds regularly overwinter. Earlier in the 1900s, Eastern Bluebirds were common in the spring and fall, and not seen at all in the winter. Then beginning in 1986, the bluebirds overwintered in small numbers. The population that overwinters has dramatically increased in the last thirty years. As already noted in this article, the winters became milder over time, and the average number of cold days (-15 degrees or below) has also decreased. There is a correlation between the summer temperature and the population of overwintering bluebirds. Summer 2004 was the coldest on record: there was a poor crop of edible fruits, and fewer bluebirds stayed through the winter–not because they did not have enough to eat in the summer, so much as there were likely fewer fruits that remained on the bushes and trees through the winter, which would have provided a more consistent food supply. The summer after, in 2005, was the hottest on record: there was an abundance of crops and fruit-bearing trees, and the amount of food to eat meant that more bluebirds overwintered. The cold, winter days are less of a deterrent to Eastern Bluebirds than the quality and quantity of food produced by the variable temperatures. Birds that overwinter are the first to the buffet of insects that arrive with the spring leaf-out.
The impact of climate change on bird migration asynchrony is especially worrisome because birds have other stressors besides climate change. For example, insecticide affects their food supply. If there isn’t enough food, even when they have adapted their migration pattern, birds do not have the strength to travel the many miles they need to their overwintering grounds. Forest fragmentation is another problem as it contributes to habitat loss, and birds need to make their home no matter where they fly. Warming temperatures are coming faster than ever and, compounded by these other stressors, we’ll see the decline and extinction of many species. The loss of even one species can have dramatic and unexpected effects on the food chain. Species loss leaves our ecosystems vulnerable to invasive species that completely change the nature of the food chain. Climate change is already affecting our local ecosystems in quiet ways that will become louder over time.
Thank you to Kim Barrett, Associate Director, Science and Partnerships, and Julia Marko Dunn, Natural Heritage Ecologist for their contributions.
Andrew, M.E., Elmendorf, S.C., Guralnick, R.P., Leyk, S., Mayor, S.J., Otegui, J., Pearse, I.S., Schneider, D.C., Tingley, M.W., & Withey, J.C. (2017). Increasing phenological asynchrony between spring green-up and arrival of migratory birds. Scientific reports.
Last modified: September 6, 2017