According to creationist definitions the Herring gull and the Lesser black-backed gull of northern Europe must be separate kinds or evolution is true and the concept of "kinds" is wrong.
The Herring gull ranges west over the North Atlantic where it encounters the almost identical American herring gull. The American herring gull can be found all across northern North America, although there are some differences in apperance from the east of Canada to the west of Alaska. Across the Bering Strait is found the Vega herring gull, and then Birula's gull, Heuglin's gull, the Siberian lesser black-backed gull and finally, back in northern Europe, the Lesser black-backed gull. Where each of these populations meet, there are known to be hybrids, except between the Herring gull and the Lesser black-backed gull.
This was formerly known as a ring species. Recent research has shown that during the past ice age the ancestral populations were actually conserved in two different reservoirs further to the south that moved north and re-encountered each other about 10,000 years ago to form the present pole circling population.
None of this is a problem for evolution but creationists tend to deny ring species.