What's the hardest European bird to see?

Almost all bird species are common somewhere. Even many endangered species have a core habitat where they are frequently encountered. But a small proportion of species are just plain hard to see, whether because they are sparsely or unpredictably distributed, skulky, or live in habitat that is difficult for humans to access.

By choice of home, I do most of my birding in Europe. I've been slowly ticking off all of Europe's regularly occurring bird species. Which got me thinking: Which European birds are the hardest of all to see? Which species are likely to be the last ones to make it onto a European birder's life list?

Of course, many species have turned up in Europe only once or twice. These 'vagrants' or 'accidentals' are nearly impossible to see in Europe, even though most are commonplace in their ordinary homelands. So let's exclude these kinds of species from consideration and focus on species that regularly occur in Europe. A good starting point, you might think, would be the European Red List of Birds, compiled by BirdLife International. But most of the species listed as 'endangered' or 'critically endangered' on the European Red List are easy to see elsewhere, even if their foothold in Europe is in jeopardy. Which European birds are most difficult to see anywhere in the world?

Sadly, two species that occurred in the European region until relatively recently -- the Great Auk and the Canary Islands Oystercatcher — are now globally extinct, while one additional species — the Slender-billed Curlew — is feared to be so.

Of the species that are definitely still with us, here are a few candidates for 'hardest to see':

​Rock Partridge: Not to be confused with its easier-to-spot cousins, the Chukar and Red-legged Partridge, this species prefers rocky mountain slopes and dry open country. It isn't especially rare: The IUCN classifies it as 'Near Threatened', with a population estimate of nearly 100 000 mature individuals. But the Rock Partridge's habitat preferences and shy nature make it hard to get a glimpse of. For at least one experienced birder, it was the last European species make it onto their life list. Birders' gen says that this species is easiest to find in the Balkans, where it can also be found in dry low-lying areas.

Scottish Crossbill: This species has a controversial taxonomic history. Supposedly Britain's only endemic vertebrate, 'Scotsbills' are intermediate between Common Crossbill and Parrot Crossbill in morphology and can only be identified safely, if at all, by their vocalisations. Recent research has shown that crossbill calls are partially learned, however, throwing into doubt the reliability of their vocalisations as basis for delineating species. If Scottish Crossbill really is a separate species (I have my doubts), it is almost certainly Europe's hardest to identify!

Monteiro's Storm-petrel: This species has a fascinating taxonomic history, belonging to a group of 'band-rumped' North Atlantic petrels that are very similar in morphology but breed on different islands. Monteiro's breeding grounds consist of a few small islands in the Azores, with a total population of only around 700 individuals. This is the European species with the second-fewest sightings on eBird (for the lowest, see below!)

Black-headed Penduline-Tit: This one might seem like cheating. A common definition of Europe takes the Ural Mountains and the Ural River as its northeastern boundary. This species occurs along the Ural River in Kazakhstan and so juuust inside Europe according to this definition. This is apparently a tricky species to see anywhere in its range, though. It is also the European species with the fewest sightings on eBird (currently just 65!), although this is probably partly because few birders visit Kazakhstan.