A species of bird lives only on two islands A and B of an archipelago.\\
At the beginning of 2013, 20 million birds of this species are present on island A and 10 million on island B.

Observations over several years have allowed ornithologists to estimate that, taking into account births, deaths, and migrations between the two islands, we find at the beginning of each year the following proportions:
\begin{itemize}
  \item on island A: $80\%$ of the number of birds present on island A at the beginning of the previous year and $30\%$ of the number of birds present on island B at the beginning of the previous year;
  \item on island B: $20\%$ of the number of birds present on island A at the beginning of the previous year and $70\%$ of the number of birds present on island B at the beginning of the previous year.
\end{itemize}