  Weakly interacting cold dark matter (CDM) particles, which are otherwise
extremely successful in explaining various cosmological observations, exhibit a
number of problems on small scales. One possible way of solving these problems
is to invoke (so-called) warm dark matter (WDM) particles with masses $m_x
\sim$ keV. Since the formation of structure is delayed in such WDM models, it
is natural to expect that they can be constrained using observations related to
the first stars, e.g., the 21 cm signal from cosmic dawn. In this work, we use
a detailed galaxy formation model, Delphi, to calculate the 21 cm signal at
high-redshifts and compare this to the recent EDGES observations. We find that
while CDM and 5 keV WDM models can obtain a 21 cm signal within the observed
redshift range, reproducing the amplitude of the observations requires the
introduction of an excess radio background. On the other hand, WDM models with
$m_x \leq 3$ keV can be ruled out since they are unable to match either the
redshift range or the amplitude of the EDGES signal, irrespective of the
parameters used. Comparable to values obtained from the low-redshift Lyman
Alpha forest, our results extend constraints on the WDM particle to an era
inaccessible by any other means; additional forthcoming 21 cm data from the era
of cosmic dawn will be crucial in refining such constraints.
