Wednesday, February 4, 2015

Flocking Robins

Flocking Robins - Kirk M. Rogers
When we first moved into Bull Creek in late winter 19 years ago we were amazed to see flocks of 20-30 robins in the field.  This is now an annual occurance as they flock together all over our valley grassland.  Unlike the starlings which cluster in our Springfield yard, the robins are widely spaced over a large field, each intent on its own territory.  When they decide to move to a nighttime roost, they depart in scattered waves, filling the air for several minutes, still separated a respectful distance from their companions.  They seem to seek the safety of numbers while remaining independent.

Robin flock on the ground - Denaple
Robins are relatively independent during the summer but form flocks in the winter.  Yesterday I counted 200+ spread over a six acre field.   They paid little attention to me until I finally drove through, then flew in groups headed to a field across the creek.  This is their valley and I am the intruder.  So how do you count that many robins, you may ask.  There is an old naturalist trick - using binoculars, count their legs and then divide by two.

European robin - Wikipedia
The American robin is actually a thrush, named a robin by early settlers for a perceived resemblance to an unrelated flycatcher, the orange-breasted European robin.  This is probably better than its scientific name, turdus migratorius, (turdus is a Latin word for thrush).

Watch a robin in your yard for a minute and you will probably see it grab an insect or extract a worm.  In the winter when insects are scarce, the Canadian robins migrate south to the Midwest, driven primarily by the need for food resources.  Now they show their frugivore tendencies, eating all available berries.  They look for soft mast and find plenty hanging on into the winter.  Some berries like our deciduous holly are bitter in the fall but "ripen" during the winter, providing food at a time when other resources are gone.  

An overgrown cedar glade behind our house provides cover and lots of cedar "berries," actually miniature cones but all the same to a hungry robin.  I suspect this is a major factor in the large flocks that we see.  But we don't have any bragging rights with only 200 birds.
"The largest flock during the GBBC was reported from Mark Youngdahl Urban Conservation Area in St. Joseph, Missouri. Observers there estimated there were 5 million Red-winged Blackbirds along with a flock of 1.5 million American Robins." Great Backyard Bird Count

Inebriated robin-
Climate change is affecting their migration as robins only fly as far south as necessary to find food.  Gradual warming may be also affecting the plant species available with time.  Expanding suburbs with ornamental planting may be another factor.  Another possible effect of the warming trend is drunken birds which may show up on Youtube.  Frugivores like cedar waxwings and robins may consume fruit that has fermented.  Longer autumns and increased freeze/thaw cycles contribute to increased fermentation so if warming continues, there may be more robin party time in the future.

Geoff LeBaron of the National Audubon Society has been in charge of Christmas bird counts since 1987. He maintains one of the largest databases in ornithology.
"Historically most of the robins wintered pretty much in the deep south/southeastern U.S. Over the last 40 to 50 years they've increased significantly, especially in the northern and eastern part of their range.  You're likely to see three or four times more robins when you're out on a Christmas bird count now than you were in 1965."
There is a lot more on robin migration and winter habits at this link.

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