If "x" is the number of European swallows, then, for every one European swallow, there would be 2x American swallows.
Thus, the total number of swallows would be 2x+x=90 swallows
Thus, the number of European swallows x=<<30=30>>30.
And the number of American swallows is 90-30=<<90-30=60>>60.
A European swallow can carry twice what an American swallow can carry, or 2*5=<<2*5=10>>10 pounds.
30 European swallows can carry a maximum of 30*10=<<30*10=300>>300 pounds.
60 American swallows can carry a maximum of 60*5=<<60*5=300>>300 pounds.
Thus, a flock of 90 birds, with 30 European and 60 American swallows, can carry 300+300=600 pounds
#### 600