If you’re an avid follower of the “Living with Change” blog (and let’s be honest: who isn’t?), you have probably read a lot about bird migration. There are so many incredible facets to this topic, as covered wonderfully by Clara Cooper-Mullin in her recent post. The great thing about bird migration in particular is how extreme it can get. A large portion of the songbirds that breed in North America winter somewhere in the tropics and then migrate to lush breeding areas throughout the lower 48 and southern Canada. We know the basic advantages of migration for birds, chief among them the chances for more food and nesting resources. But what about the birds that go way up north? Why go the extra mile (in this case, the extra 1,000 miles)? What about this far northern environment is so enticing to many migratory species? And what kind of changes is the north experiencing that could greatly impact these animals?
I was fortunate to study this very topic while working in the Canadian subarctic this past summer outside of Churchill, Manitoba. This research camp has been studying nesting colonies of Lesser Snow Geese for 50 years, and has discovered a plethora of fascinating environmental, climate, and population dynamics that have impacted the tundra significantly. The most dramatic and significant impact on this ecosystem has been the habitat degradation caused by booming snow goose populations (think Canada geese pooping and eating people’s lawns but way worse). Additionally, climate change is steadily altering the landscape on the tundra as well. Willow and birch shrub continue to creep north each year, changing habitat from grassy tundra with minimal willow to shrub-dominated river and tidal zones.
My research interest was focused on the impacts of this habitat degradation to the community of songbirds that nest in these areas. Specifically, I was interested in the nesting success and nestling growth rates of Savannah Sparrows that bred in two different northern habitats: areas degraded by geese and healthy coastal areas that have not been impacted by snow geese. Savannah sparrows are the perfect species to study in such environments. They are a relatively generalist species when it comes to nesting sites, they have multiple populations that stretch from the arctic to the Midwestern U.S., and their breeding behavior has been studied on the tundra, in grasslands, and in coastal areas of the northeast.
Given this rather wide distribution, Savannah sparrows are an excellent species to examine some of the different breeding strategies across latitudes; namely the reason to nest so far north. Apart from plenty of insects and a bounty of nesting sites, there are a few other advantages to nesting way up north, as well as some unique challenges.
Challenge: Breeding season is shorter.
If a songbird decides to nest further south, the breeding season begins around mid-May, with insect (food) abundance peaking right around when baby birds begin to hatch. Additionally, as the weather remains favorable and if insect levels are normal, the chance for attempting multiple clutches is better the further south you are. Nestlings can take a bit more time to develop, and to some extent this is limited by the amount of daylight parents have to forage for their young, as well as the extent of insect populations and competition in a given area.
As you probably already know, things are kind of cold on western Hudson Bay. And it stays colder for longer. I arrived to camp on May 27th. This is what it looked like.
Now, songbirds can handle snow for a little while, but they can’t nest in it. Also, with no leaves and no warmth, there is a lack of adequate cover for a nest and most importantly, no bugs. Songbirds are pretty flexible when it comes to waiting on the right conditions to begin building nests and laying eggs. But they can’t wait forever, and on top of that, Savannah Sparrows had to complete one helluva migration to make it to northern Manitoba. Adults that arrive on the breeding grounds have to establish a territory and find a mate. But with minimal food and a snow-covered landscape, mating takes a bit of a back seat to finding food and personal maintenance.
Average nest hatching date in 2018 was July 8th. While this number is likely to change from year to year, it won’t change by much. For Savannah Sparrows that nest in places like coastal Maine, the first nests hatch around first week of June (Wheelright and Rising 2008). This means many birds south of Churchill will have fledglings before eggs even hatch up north. This delayed timing has other implications as well. Most songbirds in the lower 48 will have multiple nesting attempts, particularly if the first one fails. Say a squirrel or blue jay ate your first clutch of eggs on Memorial Day. Oh well. You still have the whole month of June and July to build a new nest, lay some more eggs, and raise some fledglings. But if you nest way up north and didn’t lay your eggs until the end of June, there is a much bigger time crunch. Depending on when the first clutch failed, you may just run out of time to make another nest. Sure, you can try to re-nest and lay eggs in mid-July. But given the roughly three weeks needed to lay, incubate, and fledge, birds are running into a big problem with much less food available by mid-August and rapidly changing weather come September.
Advantage: The days are longer, so growth is faster!
Yes, it certainly seems like a bummer to have only two months or so to raise your young. But what a two months! Average day length in June and July in Churchill is around 18 hours, with a few more hours added to the prolonged twilight periods. And there are WAY more insects on the tundra and edges of the boreal forest than there are in the forests and fields of the lower 48 (believe me, it’s like insect Armageddon up there). So mangia mangia!! Similar species nesting in Rhode Island only have at most 14-15 hours of daylight to feed and forage during the summer. Birds up north have more time and more food available. This adds up to nestlings getting fed more often than their counterparts down south. This also means young spend shorter times in the nest, which is usually the most vulnerable and dangerous time for young songbirds. They grow up so fast…
Sometimes an advantage, sometimes a disadvantage, but always dangerous: Nest Predators
Seems like anywhere you go as a tiny songbird building a nest and laying eggs, there will be some critter that thinks your eggs are delicious. Nest predation just goes with the territory of being a songbird.
At first blush, the sub-arctic tundra has a more favorable lineup of would-be nest predators. Predators like squirrels, chipmunks, raccoons, jays, magpies, cooper’s hawks, and even deer are abundant at lower latitudes (and all will eat an egg or nestling if given the chance). Also, there is a wider variety of nesting songbirds down south, so it behooves predators to cue in on songbird activity. Up north, squirrels, chipmunks, and most other small mammals are a no-show. Canada Jays only hang out near the boreal forest and aren’t found on the tundra. Seems like a win, right?
While the variety of nest predators doesn’t quite match that of the south, danger still abounds. Short-tailed weasels roam the area and are expert nest robbers. Ravens might not spend too much time worrying about little songbird eggs when there are shorebird and duck nests to be had, but you never know with corvids. And in some years, Arctic Fox make their presence felt on the tundra. In short: building a nest and laying some eggs always comes with risk.
So why study Growth up north?
Despite the extra time and energy needed to make it up north, and despite the shorter breeding season, the Savannah sparrows of Churchill (and even further north!) have successfully carved out their little slice of heaven in the tundra scrub. What remains to be seen is how the ever changing northern climate and additional goose degradation has impacted their breeding success, specifically the growth of their young.
Goose degradation has already been shown to be detrimental to Savannah Sparrow nesting density in Churchill. My questions were pretty basic but nonetheless important in regards to breeding success. Are individuals able to successfully nest and fledge young in both degraded and non-degraded sites? The implications of this are obvious: if Savannah sparrows can’t fledge nestlings in one site over the other, these degraded areas of tundra and coastal scrub become ecological traps for songbirds.
If birds can successfully raise and fledge young, will there be any differences in the growth and development of their young? Growth and development is extremely important in the life cycle of a songbird. Not only does it impact the ability for a bird to even leave the nest and survive on its own without parents, young birds in northern environments must be able to reach the proper body condition by fall to complete (you guessed it!) their first critical migration down south.
As we start to answer these and (hopefully) many more questions surrounding growth, development, and success in northern climates, we will be able to see a clearer picture of how shifting environments impact northern ecosystems and the birds that inhabit them.
Wheelwright, N. T. and J. D. Rising (2008). Savannah Sparrow (Passerculus sandwichensis), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi-org.uri.idm.oclc.org/10.2173/bna.45
About the author:
Steve Brenner studies the impacts of habitat management on avian spatial ecology in the Scott McWilliams lab at the University of Rhode Island