"Birds and Seeds," by Brian Williams.
"It was the pine siskins that first piqued my interest. They would sometimes work at cracking seeds for minutes at a time, while the house finches rotated in and out. Why did they spend so much time on the feeder? I realized midway into a slurp of hot coffee that this was a perfect lesson in simple tools. The siskins' comparatively slender, triangular bills couldn't crack the seed hulls quickly, so they had to work patiently. The reward was too rich to pass up, and they made up for their less efficient bills with aggressiveness - flashing the stripes of yellow in their wings and tail when the finches would try to crowd them off the feeder."