From the chart above we can easily see where walnut is far stronger than birch in virtually every category that makes a difference in stocks. Because of the OP's location I compared paper birch. Sweet birch has slightly better numbers so I am using it first but adding yellow to make it easier to see both:
walnut weight 0.55 rupture 14,600 elasticity 1.68 work 10.7 impact bending 34 Comp. par. 7,580 Comp. per. 1,010 Shear 1,370 Tension Per 690 Side Hardness 1,010
sweet birch weight 0.65 rupture 16,900 elasticity 2.17 work 18.0 impact bending 47 Comp. par. 8,540 Comp. per. 1,080 Shear 2,240 Tension Per 950 Side Hardness 1,470
yellow birch weight 0.62 rupture 16,600 elasticity 2.01 work 20.8 impact bending 55 Comp. par. 8,170 Comp. per. 970 Shear 1,880 Tension Per 920 Side Hardness 1,260
Weight numbers are actually specific gravity and obviously the higher density relates to a heavier stock. Walnut weighs about the right amount...
Rupture, elasticity, and work to failure together highlight the fact birch is much springier or less stiff than walnut. Stiffness is extremely important. These measures are taken during gradual bending.
Impact bending is a measure of how much the test piece bent with sudden impact rather than the more gradual tests. Again, much stiffer.
Compression Parallel (comp par) is a measure of much pressure it takes to crush the sample, lenghtwise. Because of the difference in density there is "more wood in the birch than walnut" so it will be harder to crush. The cost for the greater crushing resistance is extra weight in the stock.
Compression Perpendicular (comp per) is the measure of crossgrain pressure required... about the same numbers yet the greater weight of the birch adds nothing to this property.
Shear is essentially the resistance to splitting under force along the grain. Birch bends rather than splitting.
Tension Perpendicular to the grain is a function of how easy it is to pull the wood apart across the grain.
Side Hardness (Janka) is the weight required to press a 1/2" steel ball halfway into the wood.
Knowing walnut's place as the acknowledged king of stock wood it is easy to see which properties are most important. Stiffness rules...
