In this task, you are given a sentence from the research paper and your task is to classify the given sentence into the following categories: Background (Why is this problem important? What relevant works have been created before? What is still missing in the previous works? What are the high-level research questions? How might this help other research or researchers?), Purpose (What specific things do the researchers want to do? What specific knowledge do the researchers want to gain? What specific hypothesis do the researchers want to test?), Method (How did the researchers do the work or find what they sought? What are the procedures and steps of the research?), or Finding (What did the researchers find out? Did the proposed methods work? Did the thing behave as the researchers expected?). 

variations were only observed in the levels of specific IgY which depended rather on the age of ducks at the time of infection than on the infectious dose.
finding
The sentence provides information about the results of a study that examined specific differences in immune responses (IgY) to infection with H5N2 avian influenza among ducks. Therefore, it describes findings or observations made by the researchers during this process, and therefore belongs to "Finding" category..