Field identification of Iduna and Acrocephalus Warblers in Europe can be tough in some cases, as plumage differences tend to be subtle between species. They often have different distribution areas or choose different habitats, but at the same time, most of them are migratory birds, and the chance of a vagrant is always present, so nothing should be ruled out when checking out a suspicious bird.
In Iberia, 3 breeding species are similar enough to potentially cause ID pitfalls. Isabelline Warbler, Melodious Warbler and European Reed Warbler. Plumage coloration differences are usually visible, but light and viewing conditions, worn out individuals or young birds can make this difference look less distinct than expected. There are, of course, other characteristics to keep an eye on –song, habitat, behavior etc.–, but birds do not always sing, and during migration they can turn up anywhere.
In Catalonia, Melodious and European Reed Warblers are common breeders and migrants. Isabelline, on the other hand, is rare, with some small populations reaching the Ebro Valley in nearby Aragon and Navarra Regions.
The following photographs of birds ringed in Southern Spain show one difference between the three species that is quite striking when seen close up: bill broadness. Isabelline Warbler has a very broad bill base, making it look slightly convex or “swollen”. Melodious Warbler has an intermediate shape and size, and European Reed has the thinnest of the three. Also note differences in color.
Of course, in field conditions, these differences are not so easy to see and, again, we are not even considering other similar species, like Eastern Olivaceous Warbler. So, if you come across an interesting bird, our advice would be, try to take good pictures of it. This is easier said than done, we know! But it would give you the chance of further detailed examination, which is both challenging and fun!