Sentence:  <WSD>Addling</WSD> accounted for 1% of clutches laid, 1% of all nests, and 2% of all complete failures. ... In most addled clutches all the eggs remained intact, but in some, one or more eggs were broken during incubation. ... [W]hile some egg <WSD>addling</WSD> was due to pollutants, other <WSD>addling</WSD> was probably due to natural factors. 
Meaning of the word marked with <WSD> tags:  The act of causing fertilised eggs to lose viability, by killing the developing embryo within through shaking, piercing, freezing or oiling, without breaking the shell or other outer layer.