Pittsburgh’s winter crow flock has moved … just a little.  No longer at Heinz Chapel they’ve now chosen the London plane trees between Schenley Plaza and Carnegie Library.

In front of the Library the air smells fishy, the sidewalks are blotched, and it’s slippery when it rains.  When folks figure out they’re walking on crow poo their reaction is “Yuk!” and then everyone wants to know, “How many crows are there?”

I don’t know. We’ll have to count.  Easier said than done!

Counting a crow roost is an unexpected challenge.  Crows prefer tall well-lit trees where they perch close together all over the top.  You can’t see them from street level because the streetlights shine in your eyes and obscure them.  Sneaky crows.

However, you can see them from above.  Peter Bell took this photo from an upper floor at the Chevron Science Center in 2011.  As you can see, the crows are well lit and countable.  The Cathedral of Learning would be a good vantage point for the Library crows.

Count them 1-by-1?  Nope!  There are far too many crows and they shuffle around.

To get a good estimate, wait until the crows settle in for the night (after 6:00pm) then count one tree full of crows, count the number of trees, and multiply.  Here’s how.

1.  Pick a typical roost tree and count 10 crows in it, circled below.

2.  Assume the 10-crow circle represents the size of 10-crow groups.  Count the number of circles that have crows in them.  See below.  (I made the circles bigger where the crows are sparse.)

3.  Multiply the number of circles by 10 to get the number of crows in the tree.  In this tree it’s 10*23 so my 1-tree estimate is 230.

4.  Now count the number of trees with roosting crows.  I think there may be 20 to 30 trees full of crows at the Library so …

5.  Multiply the 1-tree count by the number of trees.    20*230 is 4,600    30*230 is 6,900.

Before I did this exercise I guessed there were 4,000 crows at the Library.

Hmmm.

Anyone up for a challenge?  Want to count crows from the Cathedral of Learning?

How many crows are there?

(photo by Peter Bell)

p.s.  The Pittsburgh Christmas Bird Count counted 25,000 crows on 1 January 2017.  I know this doesn’t include all of them!   [My husband jokes that if counting by circles is too difficult, count the number of crow legs and divide by two.]

Meanwhile, Pitt is working to mitigate the crows’ effect on campus: https://pittnews.com/article/115672/featured/campus-crows/