Im not sure about this for all strains of HPAI. Maybe the wild birds they found, just came in from the North?possibly. if I remember well virus cannot survive 18 C and we definitely have temperatures over that.
There were no outbreaks in farms in the Netherlands since the end of March when we have more sun and temps do rise above 18°c. So there must be some truth in it.
The map shows where they found contaminated wild birds in the Netherlands in May and June.