My organism is a bird-like organism that lives on an earth-like world. My creatures speed clocked at mach 1. What material is my organism reinforced in in order to reach mach 1 just by flapping its wings so fast?
What you have described is not a bird so much as a cruise missile.
This creature evolved into a long sleek tube of metal and carbon fibre. Most of the mass is taken up by a solid fuel turbine engine. That lets it reach mach 3.
They can hit the stupidly high speed of mach 15 by holding onto the space shuttle as it launches.
Those speeds are significant. Specifying a Mach number is such an odd thing to do for something so unphysical. Whatever you make it out of, it needs to be heat resistant and your body needs to be rigid to withstand immense forces.
With Mach 3 being your average speed, consider the almost fully titanium body of Lockheed SR-71 Blackbird:
The airplane is 92% titanium inside and out.
The outer windscreen of the cockpit was made of quartz and was fused ultrasonically to the titanium frame. The temperature of the exterior of the windscreen reached 600 °F (316 °C) during a mission. [...] On landing, the canopy temperature was over 572 °F (300 °C).
This article on the Mach number mentions some other aspects to deal with surface heating:
cooled nickel-titanium skin
special silicate tiles
Hypersonic designs are often forced into blunt configurations because of the aerodynamic heating rising with a reduced radius of curvature.
Your reinforcement can be whatever strong material or lattice structure but your issue is joints. Manmade objects at those speeds are rigid bodies. Organic articulated joints, muscles and tissue will likely not withstand the forces acting on an object travelling through a medium at those speeds.
Some other aspects to consider:
- How does the creature keep its configuration?
- How does it keep its eyes open?
- How does it breathe if the air that enters its lungs and body is 300+ °C?
- What keeps the water in its body from boiling?