Charles and Mary Brown have demonstrated that as colony size increases for cliff swallows, the ...

Charles and Mary Brown have demonstrated that as colony size increases for cliff swallows, the number of nests with one or more unhatched eggs decreases.
 
  They have also demonstrated that as the size of colony increases, the number of parasites increases, and these parasites directly cause nest failures and juvenile mortality. These two observations indicate that there are trade-offs in group living. What would be a better measure than unhatched eggs to understand the benefits of group living in cliff swallows?
 
  What will be an ideal response?