I feel like I’ve spent the last week swimming in a very, very big lake of migration information. I think I may have made my goals for this month a little too broad given the sheer volume of data out there- maybe I should have concentrated on a specific area or genus of birds. Trying to really understand what it all means has felt almost overwhelming at times, but I feel like I’m starting to understand what migration entails and which trends have become more apparent over the last few decades.
First of all, migration is tough. There are distances to cover, predators to avoid, and all sorts of variables that can change. More than 2/3 of the breeding landbirds in temperate North America spend their winters farther south (Keast & Morton 1980), so many species have to overcome a variety of obstacles. Moore et al. (1990) found that there were 3 main sources of mortality among birds crossing the Gulf of Mexico: death during flight, starvation upon arrival at a stopover point, and predation by raptors. Many of those birds started their crossing at night, navigating in the dark. As weather conditions deteriorate, such crossings become even more difficult. Birds crossing the North and Baltic Seas off Germany tended to fly at lower altitudes in bad weather, increasing their chance of colliding with wind turbines and other man-made structures (Huppop et al. 2006). When things get really bad, it can be catastrophic: Cold temperatures and wind plus heavy snow near Lake Huron on the night of October 10, 1906 resulted in the discovery of an estimated 5,000 carcasses per mile of fall migrants along the lake shore (Saunders 1907). Oh, the avianity.
Pied Wagtail, summer migrant in some parts of Europe and Asia and year-round resident in others
Some overall trends for migrating populations have emerged, although it can be really complicated to tease out the factors contributing to those changes. In North America, there is the added complication that human impact on the landscape since European settlement doesn’t leave us with much of a baseline for long-term assessment. (Where I live in central NH, it’s largely forested today, but much more of the land was pasture and meadow 150 years ago, and before that it was forested, but not with the same structure- bird (and other) species react to those changes, but we have only general understanding of the original starting point.) Migrating bird populations in Europe and North America have seen declines, but the feeling is that there are different causes (Newton 2004): drought and desertification in the Sahara seem to be contributing to European declines, such as a drought in the Sahel which coincided with a 70% drop (ouch!) between 1968 and 1969 in the Common Whitethroat community of the UK, while birds in eastern North America that nest in forest interiors seem particularly hard hit. At the same time, North American song birds that depend on grasslands have seen significant declines over the last 35 years (Blancher 2003).
How are both grassland and forest species declining at the same time? Shouldn’t one go down as the other goes up? Well, it all goes back to changes in land-use, although the chain of events can differ. In the case of grassland-dependent birds (536 species in North America use some grassland, 42 are heavily dependent on it- Blancher 2003), loss of grassland through conversion into agricultural land and other uses (for example, housing developments, golf courses, etc.) means that there is simply less of an important resource to go around. If you are a territorial bird, that’s an even bigger issue because you may be prevented from even getting near the resources you need. And remember that these birds move around each year, so losses in wintering habitat can cause declines even when the summer habitat is holding steady (Dolman & Sutherland 1994). For birds that nest in forest interiors, there are other issues. As forests become increasingly fragmented, once again through development, you have what is known as the edge effect: the ratio of edge to interior increases. Why is this important? Nest predators such as raccoons and house cats are more common in edge habitats than in the forest interior, so those birds and their chicks are at greater risk. They are also at greater risk from nest parasites, such as Brown-headed Cowbirds which many birders love to hate, who lay eggs in other birds’ nests and whose chicks may out-compete the host-parents’ own young for food.
Chipping Sparrow in Lincoln, NH- but Mirka, you may say, I see Chipping Sparrows year-round at my house- yes, but are they the same Chipping Sparrows? These birds are short-distance migrants, so they travel, but some only go to southern North America
A further complication is provided by global climate change. With some species, researchers have noticed earlier arrival dates in breeding habitat and earlier nest initiation. (I’m not going into all of those details here, but check out Just Fascinating Stuff for a comparison of historical arrival dates for a handful of species in MN and Manitoba- some of those changes were a little scary…) Birds may start arriving in breeding locations before their food does, creating a resource mismatch that could negatively impact breeding success (Leech & Crick 2007). Species that migrate shorter distances seem to be responding more in terms of early arrival, and it’s possible that some may eventually stop migrating (Butler 2003), as has already happened with Blackcaps in the UK and White-throated Sparrows in NY’s Finger Lakes area.
So, that’s an overview of migration and the trends that have emerged within the last few decades. This big-picture stuff, and a wise person once told me not to look at the big picture for too long, lest I get overwhelmed, so next week I’ll be looking at a few specific birds. I want to know what their current situation is and what, if anything, is being done to remedy problems. If there is a particular migrating song bird that you are interested in, let me know and I’ll do my best to provide a run-down.
Works cited:
Blancher, P. 2003. Importance of North America’s grasslands to birds. North American Commission for Environmental Cooperation: Birds Studies Canada.
Butler, C.J. 2003. The disproportionate effect of global warming on the arrival dates of short-distance migratory birds in North America. Ibis 145: 484-495.
Dolman, P.M. and W.J. Sutherland. 1994. The response of bird populations to habitat loss. Ibis 137: s38-s46.
Huppop, O., J. Dierschke, K. Exo, E. Fredrich, and R. Hill. 2006. Bird migration studies and potential collision risk with offshore wind turbines. Ibis 148: 90-109.
Keast, A. and E.S. Morton. 1980. Migrants Birds in the Neotropics: Ecology, Behavior, Distribution, and Conservation. Washington, DC: Smithsonian Institution Press.
Leech, D.I. and H.Q.P. Crick 2007. Influence of climate change on the abundance, distribution and phenology of woodland bird species in temperate regions. Ibis 149:128-145.
Moore, F.R., P. Kerlinger, and T.R. Simons. 1990. Stopover on a Gulf Coast barrier island by spring trans-gulf migrants. The Wilson Bulletin 102(3): 487-500.
Newton, I. 2004. Population limitation in migrants. Ibis 146: 197-226.
Saunders, W.E. 1907. A migration disaster in western Ontario. The Auk 24(1): 108-110.