Elaborating on previous hints: suppose a=|AB|, where A and B denote the positions of two birds, with bird A flying towards bird B, and the tetrahedron edge AB serving as the x-axis. Then \frac{\mathrm d a}{\mathrm dt}=-v_{Ax}+v_{Bx}, where v_{Ax} and v_{Bx} denote the projections of the corresponding birds’ velocities onto the x-axis.
Although it is preferred that you solve the differential equations analytically using your own skills, if you are unable to do so, feel free to (a) use online tools (such as Wolfram Alpha) or (b) integrate the equations numerically.
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