Sentence:  She fell asleep leaning on her hand , hearing the house creaking as though it were a living a private life of its own these two hundred years , hearing the birds rustling in their cages and the <WSD>occasional</WSD> whirring of wings as one of them landed on the table and walked across the newspaper to perch in the crook of her arm . 
Meaning of the word marked with <WSD> tags:  occurring or appearing at usually irregular intervals