for this you have to multiply the two numbers :
\(\frac{3}{28}\) are birds and out of those \(\frac{4}{15}\) are there relevant birds.
Explanation why you have to multiply:
in a similar situation, if you have a number and you want to find out 3/4 of this number: you multiply the original number by 3/4. Here it's just confusing that both numbers are fractions.
\(\frac{3}{28} * \frac{4}{15}= \frac{3*4}{28*15} = \frac{3*1}{7*15}= \frac{1*1}{7*5}= \frac{1}{35}\)
so the result is
\(\frac{1}{35}\)