Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [16]
The spruce trees were still heavily laden with enough cones to make the tops of the trees look brown. However, while spruce cones stay long on the trees, the seeds fall out of them as the cones dry and the bracts curl out. A month earlier spruce seeds were strewn about on the fresh snow and chickadees were hopping on the snow foraging for these shed spruce seeds, while red-breasted nuthatches were picking them out of the cones in the tops of the trees. I found one spruce tree with several hundred cones under it on the snow. These cones had been chewed off and dropped by a red squirrel.
Warmed by the sun they had then melted into the snow. So this squirrel had not been in any hurry to gather them up after dropping them.
Red squirrel.
Impulsively I picked up a handful of the cones. All had only the first few bracts chewed off near the base of the cone. The snow surface was littered with cone bracts; the squirrel had been feeding on the cones as it was harvesting them up in the tree. But why did it just drop and leave most of these cones unopened? Back at the cabin later I examined five of these discarded cones. Each had on average 40 bracts, and a full cone has 2 seeds under each bract. Thus, a full cone produces about 80 seeds. The five cones could contain close to 400 seeds, but I found instead only a total of 23, or about 5 seeds per cone remaining. No wonder the squirrels had dropped the cones after sampling them, or not gathered them up later.
How many seeds must a cone contain before a squirrel decides it is not worth the effort to invest more energy and then discards it? A squirrel can eliminate the possibility of sampling the same bract twice, because it must chew each bract off in order to see what’s under it. However, crossbills cannot know for sure which of the 40 bracts on any one cone it has sampled has seeds under it. Since it can’t very well memorize which bracts have been sampled and which not, it likely can’t help but sample some bracts more than once. It may therefore need to have fairly full cones in order to make the hard work of separating the stiff bracts from the cones worthwhile. On the other hand, prying bracts apart might be easier than the squirrels’ way of chewing them off. Had the crossbills left here because the cones had by now shed too many seeds?
Closed.
Hemlock cone.
Red spruce cone open and shedding seed.
This was not a question I could hope to answer, but it did induce me to climb to the top of a spruce tree in order to retrieve a twig laden with cones. The bracts of the cones were opening up, and when I banged limbs that had cones on them I caused showers of seeds to twirl down. I examined ten cones, having 40 to 50 bracts each, finding that seed number per cone (20, 6, 36, 8, 2, 35, 28, 16, 12, 17) was considerably higher than on my previous count of squirrel-discarded cones. Was this an unusual tree, or one that by chance crossbills had not visited? I was obviously not going to answer this question just now, either. But it was a fabulous day with deep blue sky, rising temperature, and the crust was still solid for excellent walking on snowshoes. I’d better make the most of it.
Walking on, I eventually found two other spruce trees where squirrels had been feeding on cones. Ten out of 321 had only a few bracts eaten off at the base (where a squirrel always starts) and the rest of the cones were then left. That is, they were discards.
But seed number per bract