Change in arrival from year to year I am using to Excel to analyze a dataset I have. I'm looking at bird migration. I have the date the birds were seen and the number of birds (abundance). I have a dataset of 10 years and I am trying to see if there is a shift in arrival time from year to year. What would be the best way to approach this?

An idea is split the 10 years of data into 10 separate datasets which span a single year. Then, only use the portion of each year where the bird population is increasing. Perhaps you just select a date range to use in each year. With these 10 sectioned data sets, divide them by their maximum value such that they are normalized (only range from 0 to 1). Now, use logistic regression (fitting tool may be available in excel) to fit the 10 data sets separately. The function you are fitting looks like this:
$$
a(d) = \frac{1}{1 + e^{-(d-\mu)/s}}.
$$
The fitting tool should provide a $\mu$ and $\sigma$ to make the above function fit each of your data sets.
$\mu$ roughly indicates the value of $d$ where your bird behavior has "transitioned". You can plot $\mu$ for each of the 10 years to see how it has changed!
