On April 26, 2014 I accompanied Karl Werner, Rob Rutledge, and Mike Clay to survey the White River Heron Rookery in NW Johnson County. The rookery is on CILTI property (Central Indiana Land Trust Incorporated) on the west side of the river and is not open to the public. The goal is to census the Heron Rookery every five years. The census should have taken place last year, but high water prevented the count.
The day was perfect for the census. We surveyed the rookery in mid-afternoon under clear skies which was good since the temperature reached 80 degrees Fahrenheit. That is important since some of the Great Blues fly off the nest uncovering the eggs. But the warmer temps should have kept them warm for the short time they were exposed.
We worked quickly going in a north-south pattern marking the counted trees so we wouldn’t tally them again. We probably weren’t in the main part of the rookery for more than 45 minutes.
Listed below are the results of this and previous years survey:
So why the increase and then decline in the number of nests over the years? I turned to the internet to see what others have speculated on the declines of rookeries. Some of the reasons I found were disturbances in food source, water pollution, eagles in the area taking eggs, a nearby loud noise, or simple an overpopulation of the area.
One of the things I thought about was the growth of the city of Greenwood which lies to the east. Below is the population growth of Greenwood.
1980 19,327 62.8%
1990 26,265 35.9%
2000 36,037 37.2%
2010 49,791 38.2%
I have only lived in the area a short time, so I don’t know if the city’s expansion effected the herons or their food supply. But last year I was out observing the rookery from the east side of the river. The owner of the only house in the area stopped by to talk. He said the house had been his grandfather’s so he had been coming to the area for years. He remembered as a child the herons flying all day long to somewhere “east” and then returning. Presumably with food. Over the years he had watched the number decline with greater decreases over the last 5-10 years. He speculated the expansion of gravel pits in the area might have something to do with it. In the couple of hours I spent there I only saw four herons flying east and then back. So I can’t think that the city’s growth couldn’t have impacted the rookery in some manner.
As for other reasons listed, yes there are Bald Eagles that live on that stretch of the river. The large gravel pits have loud, noisy machinery. Maybe the composition of the White River has changed.
My theory, which I have no proof or facts to back up, is there got to be too many herons for the local food source. There would have to be over 1000 herons if there were 562 nests, plus the young. There would have had to been a large supply of food to support that many herons. So hopefully many of the herons moved on to other locations with a better food source.
But like most things I have encountered in life, it is probably a combination of several things.
It would good to hear from others about the rise or decline of other heron rookeries.
Thanks to John Castrale for providing the data on past surveys.