Thursday, February 20, 2014

Why Migratory Birds Ignore Puxsutawney Phil

I for one am tired of the cold. Don't get me wrong, this winter in Delaware has been great. We've had an abundance of snow and few days of freezing rain, but this Texans had enough. I was raised on a steady diet of 6 months of hot, 2 months of cold, and 4 months that are only kind of hot. By mid February it's time to start buying swimsuits and stocking up on suntan lotion.

Punxsutawney Phil the Perpetrator
  (Alessandro M.)
Unfortunately, I don't live in Texas anymore. Punxsutawney Phil, the infallible weather muse, has predicted six more weeks of winter for the east coast. We'll forget for a second that our rodent friend is less than stellar at prediction and take his words at furry face value.

Just 2 weeks from now, the lucky residents in Texas will herald their first of the season main land migrants, and in 3 weeks every gulf state from Texas to Florida will see their very first migratory species. This may sound early to some of you, and indeed it is, but certain individuals in a population will try to get an early start to their breeding season. These include the southern breeding species like the Golden-cheeked Warbler, southern U.S wintering warblers like the Northern Parula , the early gulf migrant Louisiana Waterthrush, and the seemingly always migrating Purple Martin.


This winter has been brutally cold. We've seen record snowfalls in the south, and the great lakes ice coverage at it's highest extent since 1994 (90%). If this years weather and our furry friends ominous prediction is right, these early birds will be in a heap of trouble. So why would an individual migrate early when its still cold and how do birds cope with the wildly inconsistent weather in spring?

The extent of the ice on the great lakes this year (noaa.gov)
Lets start of first with an easy question:
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Why even show up early?
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Birds benefit from arriving early in a variety of ways. They get the best pick of territories, avoid predators, and have more chances to succeeded. All of this equates to better breeding success (the chance of successfully raising chicks to adulthood).

Males arriving to a breeding site first get the best pick of territories. If these males choose the highest quality habitat, they'll have an easier time feeding their nestlings. This increases not only an individual chicks chance of surviving, but increases the numbers of chicks a pair can raise.

Common predators for nesting songbirds include snakes and raptors (birds of prey). The time of the highest nest predation coincides with the middle of the breeding season. Nesting hawks have to forage more as their own nestlings grow up. This increases predation as the seasons progresses. With the progression of seasons, temperatures heat up, leading to more snake species migrating out of their winter dens. This all culminates in peak nest predation in the middle of the breeding season. Earlier migrants can skip this dangerous phase by nesting earlier before these factors peak.

Most songbirds nest multiple times in a season until they finally succeed. Some crazier birds may even renest after they've succeeded in raising the first nest.  In the case of Willow Flycatchers, some parents begin nesting before the first brood is even able to fly! So, with an early start to the breeding season, you get not only more chances to succeed, but more time to raise even more young!

It's clear, there's a real advantage to showing up as early as you can.
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What could possibly go wrong?
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Well mainly weather.
The top weather-related killer of birds is the cold. Untimely snow storms routinely kill birds who aren't prepared. Particularly wet snow can quickly saturate a birds feathers, forcing it to land freezing and wet. Situations like this kill even hardy snow birds. It's no surprise that species who aren't used to cold temperatures regularly die of exposure when an unseasonal cold front penetrates into the south [1].

Ice can also be particularly tricky for birds. Waterbirds, like ducks, require open water to feed. When all available water is frozen over, many starve before they can find water. Because of this, ducks often fly hundreds of miles south in search of open water in the winter. This permanent relocation leaves many of us in the Northern States (yah I just said that Auriel) with a lack of ducks for the rest of winter.

Surf Scoters, Indian River inlet, DE
Birds who usually experience extreme weather are well adapted to it. It's when temperate birds run into this weather that there becomes a problem. These birds spend the majority of their time in warmer locations and tremendous amounts of energy flying thousands of miles north to breed. These exhausted tropical vacationers are ill suited in maintaining their internal temperatures. In spring, spending a night below 10 degrees Celsius (50F) costs a songbird the same energy as flying for 3 hours. [2]. To a small song bird that's ~90 miles! [3].

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So what keeps them from showing up early?
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The range map of Golden-Winged Warblers
Migratory birds react strongly to temperature. The correlation between the average temperature at a migrants wintering grounds and summer grounds is both fascinating and complicated (We'll go indepth on this at a later date). Many studies have shown some species won't migrate north unless it's certain temperature.[2][4] Unseasonably cold temperatures in spring are usually brought about by a cold front from the north. This all but guarantees the northern latitudes are also cold. Migrants can dictate their future weather patterns by monitoring temperatures and delaying their departure till situations improve.

The pattern of migration is generally where southern migrants move through first and northern populations arrive later.  This keeps migrating birds at a manageable latitude, but can lead to highly incongruous breeding seasons. A warbler breeding in the southern Appalachian Mountains in late April has entire month head start over it's twin breeding all the way in Canada in June.


Despite evolutionary traits, some birds still fly straight into bad weather.

Last year, a late arctic blast ran straight through the Midwest. Migrating rails, already on their journey, were faced with frozen wetlands with no where to go. Lucky for them, these hardy wetlands species have adaptations to survive in conditions similar to this. Rail species like the Virginia and Clapper rail are routinely found all the way into Delaware and New Jersey in winter. The northern most breeding rail, the Yellow Rail, breeds just shy of the arctic circle. Their bigger bodies, water resistant feathers, and terrestrial foraging allows them flexibility when it comes to surviving temporarily in variable temperatures.

Two Clapper Rails taking the ice in Delaware raily well
If a birds particular tolerance or adaptations fail, we unfortunately find large scale mortalities.  In March 1904, an estimated 1.5 million birds were found dead buried in the ice and snow after a blizzard in Minnesota. In 1964, 100,000 king eiders died when water ways refroze in the spring. This impact can sometimes be quite extreme on populations. A study populations of cliff swallows in the great plains, lost 54% of their population when unseasonable weather hit the region in 1996 [1].

This extreme consequence acts as a natural checks and balances system for  birds. Nature maintains a hard line that punishes birds that arrive at the wrong time.  Without it, birds would casually move their timing earlier and earlier.

Interestingly, we're actually seeing migratory timings shifting earlier in the spring. Over all global warming trends have lead to earlier plant blooming and migratory bird timing. We're seeing not only earlier migrants but northern range expansions of sub-tropical species into the north. Clearly the cold hasn't stopped them. [5][6].

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Migratory timing is a large field in science, and this article has just touched the tip of the iceberg. We haven't for example mentioned birds being tied to high concentrations of bug activity, or the consequences of freak storms (hurricanes and tornadoes) on migrating birds (my masters research ;)). Clearly there's a lot for us to get to and always more to learn.

If you have any questions or would like to suggest a topic we cover, feel free to email us at naturalausterity@gmail.com

-Boone
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[1]Newton, I. (2007). Weather-related mass-mortality events in migrants. Ibis, 149(3), 453–467. doi:10.1111/j.1474-919X.2007.00704.x
[2]Wikelski, M., & Tarlow, E. M. (2003). Costs of migration in free-flying songbirds. Nature, 423(June), 2003.
[3]Alerstam, T., Chapman, J. W., B├Ąckman, J., Smith, A. D., Karlsson, H., Nilsson, C., … Hill, J. K. (2011). Convergent patterns of long-distance nocturnal migration in noctuid moths and passerine birds. Proceedings. Biological Sciences / The Royal Society, 278(1721), 3074–80. doi:10.1098/rspb.2011.0058
[4]Gauthreaux, S. A. (1991). The Flight Behavior of Migrating Birds in Changing Wind Fields : Radar and Visual Analyses. American Zoologist, 31(1), 187–204.
[5]Gordo, O. (2007). Why are bird migration dates shifting? A review of weather and climate effects on avian migratory phenology. Climate Research, 35, 37–58. doi:10.3354/cr00713
[6]Marra, P. P., Francis, C. M., Mulvihill, R. S., & Moore, F. R. (2005). The influence of climate on the timing and rate of spring bird migration. Oecologia, 142(2), 307–15. doi:10.1007/s00442-004-1725-x


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