There were some birds sitting on the fence. 4 more birds came to join them. If there are a total of 5 birds on the fence now how many birds had been sitting on the fence at the start?
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There were some birds sitting on the fence. 

This means that there was at least 1 bird sitting on the fence. 

4 more birds came to join them. 

This means that the total number of birds on the fence is now 5. 

So, the answer is that there was 1 bird sitting on the fence at the start.