[xcorr2()] Counting elements using autocorellation

Hello everyone,

I'm trying to use the xcorr2() funtion to do an autocorrelation in order to count the number of objects on a picture. For example this one:

However, the result is not the one I expect. Here is the result of the autocorrelation:

There seems to be a sort of symmetry and a prominent central correlation (big spot in the middle) and I can't find out how to get rid of it... (even by looking further in the definition of autocorrelation)

Here is my code:

imageRGB = imread('birds.png'); 
imageGRAY = double(rgb2gray(imageRGB));
im = imageGRAY - mean(mean(imageGRAY));

icorr = xcorr2(im); 

image(icorr);

Thanks in advance for your help!