the form of traveling bands [@Chate2008; @Solon2015]: the emergence of order in Vicsek models is followed by spatio-temporal heterogeneities.

The ability to generate self-sustained structures constitutes a generic, distinctive feature of active systems. Indeed, the formation of structures, or patterns, driven by activity is not always associated to the emergence of collective motion. Isotropic self-propelled particles, with no alignment interactions, show a tendency to aggregate into dense clusters [@Theurkauff2012; @Fily2012; @Levis2014; @Soto2014; @Liebchen2015], eventually triggering a macroscopically phase separation in the absence of attractive forces. This so-called Motility-Induced Phase Separation (MIPS) is mainly understood as arising from the mere competition between excluded volume interactions and self-propulsion, as discussed extensively in the context of Active Brownian Particle (ABP) models [@CatesRev; @Fily2012; @Redner2013; @Stenhammar2013; @Stenhammar2014; @Levis2017].

However, any experimental realization involves more intricate effects and interactions than purely velocity alignment rules or excluded volume. Consistently, experiments usually show more complex structures, or patterns, probably due to the interplay between different interaction mechanisms. Despite the ubiquity of excluded volume interactions, which should be present in any system of polar active particles - whether is wet, e.g. suspensions of micro-swimmers, or dry, e.g. animal flocks [@MarchettiRev] - its impact on the flocking transition and 