The birch seed diet

[I find that right now I have two different strands that I want to write about. There are still stories left to tell from Tanzania, but here now in Maine it is deep winter, and that is both beautiful and timely. So I think I will oscillate between the two for the time being. Today, winter prevails.]

When snow is thick on the ground, food is scarce.  The Slate-eyed Junco, Junco hyemalis,  is a tiny bird, yet it somehow survives through the long hard Maine winter.

Some food sources are easy to see, like these apples rotting away on the tree:

Others are so tiny it is hard to imagine that they provide enough food even for a junco: this one is feeding away but on what?

A large area of snow beneath two gray birch trees was covered in what looked like wood-shavings:

These are the seeds of the birch tree, whose long slender catkin has blown down in the storm, and has been swiftly dismantled by the juncos:

Each seed is minute, with tiny wings, and two little antennae:

The gray birches growing a few yards down the stream have shorter fatter catkins,

and seeds with smaller wings (this seems to be a double-yolker!) :

They gobbled these up too.

The seed is about 2mm across, and 2000 seeds weigh about one gram. The amount of nutrition in these seeds is about 5 Kcal/g, and a junco needs 28 Kcalories per day to survive in the winter, so it needs to eat more than 5g of these minute seeds, or 10,000 seeds. Of course, its diet is varied, and other foods offer more calories, but you can see why it never stops eating. To get my 1500 Kcalories a day I would need to eat 300g of these seeds, or 600,000 seeds. I think I prefer good bread, good cheese, and a glass of red wine.

Sources: 

The calorie data on birch seeds is based on white birch seeds, and is taken from Grodziński, W., & Sawicka-Kapusta, K. (1970). Energy Values of Tree-Seeds Eaten by Small Mammals. Oikos, 21(1), 52-58. doi:10.2307/3543838

The calorie data on juncos is taken from Seibert, H. (1949). Differences between Migrant and Non-Migrant Birds in Food and Water Intake at Various Temperatures and Photoperiods. The Auk, 66(2), 128-153. doi:10.2307/4080440

I am responsible for the maths!

PS How to weigh a junco, from a 2020 article in Nature on the work of Pamela Yeh:

” A dark-eyed junco being weighed: “We have a digital scale,” says Pamela Yeh. “We’ve put a bird head-first in the cup, and it’s dark in there, so he doesn’t move very much.”