In cell J$2,$ get the unique Areas.

In column K$,$ rows $2$ to $9,$ use AVERAGEIF$()$ to calculate the average count of Brush$-$footeds by Area. Pass it the area data as absolute addresses, the unique areas, and the Brush$-$footed count data as absolute addresses.

In column H$,$ join the average counts back to the original dataset. VLOOKUP$()$ takes $4$ arguments. It needs the Area from column A$,$ the data range of the table you just created $($J$2$ to K$9)$ as absolute addresses, the column in the that table that contains the averages $(2),$ and whether or not that table is sorted $($FALSE$).$

In column I, IF$()$ column G is blank, take the value from column H$,$ else take the value from column G$.$