Does anyone know what makes birch trees give sap

We are on what looks like the tail end of our maple season 60 deg forcasts and 32+ degree nights. We decided for fun to tap 25 birch trees today. Most of them are Black birch with a few yellows mixed in. Maple sap ran well once everything thawed out this afternoon but the birch taps are all dry. Does any one know when they would start running and possibly a scientific explaination of what makes a birch tree give sap? This is my forcast for my bush. http://forecast.weather.gov/MapClick...2=-74.6808&e=0 Does anyone know if these are the right conditions? Red maples will bud at any moment and the birch buds look like it is still winter. Thanks.

Birch sap will start to flow after you see the 1st butterfiles and the ground starts to thaw. For us when the creeks & low spots fill up with water it is time to tap. THat is usually after maple finishes but ther can be some overlap.
We tap 3-4 trees & when they are giving you 2 + liters/day ( 1/2 gal) you tap the remainder. If you tap when they are dry you will only get a fraction of the yield. Trees will give you an avg of 4 liters/tree/day over the season (1gal), but it can vary up to 2 + times per day.
Remember that you are dealing with a different kind of tree, different sugars & a different end use.
Birch can be 100 or 120 to 1 & you only need to finish it to 60 deg brix , not 66 like maple.
We do sell a production maunal which I'll put back up on the site shortly
Best of luck

Thanks for the info. The ground never really froze all that much so it is just mud around here. I noticed that the taps were a little wet today so maybe we are heading there. I had another question. I was going to put some that are close together on tubing. I see most everyone in the industry uses buckets. I just wanted to make sure there wasn't any reason that tubing wouldn't work.

As Birch trees here usually grow in clumps we tap many of our trees using T's & link them into a single larger bucket with tubing. We use food grade pails from grocery stores or restaurants. We put the lid on & drill a hole for the tube in the top side of the bucket. It works well to keep out water/rain/bugs etc.
When the trees are all flowing well & giving > 1 liter/quart/day then tap whatever your need. It should work fine.