crow aggregations

Occasionally eastern New Orleanians have mentioned that at certain times of the day they feel like they are on the set of Alfred Hitchcock’s “The Birds.”  The very large aggregations of black birds they are seeing is the result of a roost located nearby consisting of literally tens of thousands of fish crows.  There are two factors which probably contribute to the crow’s choosing the neighborhood that they have.  One is that the eastern part of the Crescent City contains the only remaining moderately sized (50-100 acres or more) sections of relatively isolated woodlands.  The other reason is that there are several land fills nearby and the crows find them lucrative resources.  But the real question is why do we see these enormous gatherings only in winter?  Why not year round?  The answer is quite simple: heat conservation and the resultant decrease in energy expenditure.

The birds come together in dense roosting areas each night sitting closely and sometimes on top of one another.  During their inactive period at night, the muscles are providing virtually no heat, so any and all heat retention is vital.  As day breaks, the birds become active and leave the roost to gather food.  Metabolic heat helps them through the day, but extremely cold or overcast days may keep the birds roosting.  In fact, the roost is not fool proof - many birds do not survive our relatively mild winters.  On days when activity is normal, the birds begin returning to the roost about an hour before dusk.  During mid-winter, this activity begins around 4 p.m.  The birds circle above the roosts as if to generate just a little more heat for the night.  The flock of birds grows until the sky is partially blackened by their numbers and then they rapidly settle in for the night.

As spring approaches, the roosting aggregation will slowly become smaller as birds begin to leave for a less colonial existence during the warmer months.  Breeding, rather than heat retention, becomes the dominant concern.