  The classification of galaxies as star forming or active is generally done in
the ([O III]/Hbeta, [N II]/Halpha) plane. The Sloan Digital Sky Survey (SDSS)
has revealed that, in this plane, the distribution of galaxies looks like the
two wings of a seagull. Galaxies in the right wing are referred to as
Seyfert/LINERs, leading to the idea that non-stellar activity in galaxies is a
very common phenomenon. Here, we argue that a large fraction of the systems in
the right wing could actually be galaxies which stopped forming stars. The
ionization in these "retired" galaxies would be produced by hot post-AGB stars
and white dwarfs. Our argumentation is based on a stellar population analysis
of the galaxies via our STARLIGHT code and on photoionization models using the
Lyman continuum radiation predicted for this population. The proportion of
LINER galaxies that can be explained in such a way is however uncertain. We
further show how observational selection effects account for the shape of the
right wing. Our study suggests that nuclear activity may not be as common as
thought. If retired galaxies do explain a large part of the seagull's right
wing, some of the work concerning nuclear activity in galaxies, as inferred
from SDSS data, will have to be revised.
