16th December 2019 Daniel Roca

Mapping rare birds: Where do they turn up?

Llegiu aquí la versió en català

Isabelline Shrike

Birds appearing unexpectedly far away from their regular range have delighted birders and puzzled ornithologists for a long time. The former may want to add a new species to their personal list or enjoy the exceptional sighting. The latter try to understand this complex phenomenon, that sometimes can be explained, and other times just leaves us scratching our heads. But, what is exactly a “rare bird”? Are they all the same? There is a great difference between an unusual yet, in the end, more or less regular migrant (like Red-necked Phalarope) and a true drifter (like Red-flanked Bluetail). Eastern vagrants, irruptive wintering species, southern spring overshooters… every species is different and this is what makes “rare birds” so intriguing. Let’s remark this diversity with some examples.

The case of the Rook and the Hooded Crow in Catalonia illustrate quite well why the dividing line between a rarity and a regular species can sometimes be blurry. Historically, both species have been very rare, until recent breeding events that seem to indicate the beginning of a colonization. For these two species, only older records are considered. Take the Corn Crake: Catalonia lies in the path of its migration route between Africa and Northern Europe, but the combination of birds largely overflying the region, the fact that it is hard to spot in the field, and also an overall scarce species, means you probably won’t see any. Cryptic species, like the Iberian Chiffchaff, may be commoner than we think, but are often not reliably identified in the field unless they can be heard, good pictures are obtained, or they are trapped for ringing and can be examined in hand. Some species, even though breed regularly in areas not far away from Catalonia (e.g., Rufous-tailed Scrub Robin) have a very low number of records. By contrast, other species with a much more remote distribution, like some Siberian warblers, have become more and more common (or less and less rare) in recent years, especially the Yellow-browed Warbler, which has the highest record count of all the species treated here, and is no longer considered a rarity by some (even though it breeds in Siberia and winters mainly in South-east Asia). The improving identification skills of birders actively looking for them may explain –at least partially–, the increasing number of records of this species. Some Central/Eastern Europe passerines have extralimital migration routes in normal conditions, but close enough so that it only takes a combination of right winds at the right time to bring some to our coasts (e.g., Collared Flycatcher). For some other species, Catalonia is situated at the western edge of their migration route (e.g., Red-throated Pipit) and, while very scarce, they are usually not considered true rarities. American/arctic waders, with their amazingly long distance migrations, can also appear on our wetlands almost any time of the year, and oceanic seabirds sometimes enter the Mediterranean. Finally, some native and resident species have been brought to the brink of extinction and have become very rare (e.g., Dupont’s Lark), but not in the sense of “extralimital”, “sporadic” or “accidental”, which is the one considered here. The Great Bustard, on the other hand, is now completely extinct in Catalonia, and has become, sadly, a rarity in its own right. The examples could go on. In the end, the definition of “rare” or “rarity” is not so straightforward.

And so, if every species is different, why put them all in the same basket? Well, mapping a large collection of rare bird records to see where they turn up lacks a scientific value, but it reveals some interesting facts about the studied area, as well as the birding community of that territory. In Catalonia, the website Ornitho.cat, managed by the Catalan Ornithological Institute, gathers the majority of bird records generated by hundreds of birders. Over the years, this means a lot of data. Although not all rare bird records are found there –some observers use eBird, and some historical records have not been compiled into the database–, for the purpose of this article, the amount of information is more than enough.

  • A total of 1.819 rare bird records from Ornitho.cat, belonging to 118 species (see the list at the end of the article), have been processed to prepare the following maps. These records cover mostly the past ten years (from 2009 to July 2019), with the addition of some earlier records. Records pending verification have not been included.
  • Long staying birds, often with many records over time, are represented by the first sighting only.
  • Sometimes, rare birds show up in pairs or small groups, but here they are all considered as one sighting.
  • All records are set on a 1×1 Km grid, as well as some hotspots or regular birding areas, which are selectable from the web.
  • As some spots hold more than one record, the total number on the map is 911. Spots with only one record are the smallest, and size increases proportionally to the number of records.
  • 638 spots have only one rare bird on record. The rest have at least two, with largest dots holding 11 or more rare bird records.
  • The single spot with the highest number of records (in the Aiguamolls de l’Empordà wetlands) holds 43 rare bird sightings.
  • Interestingly, 273 spots hold two or more rare bird records, indicating a good number of areas which are either very good for birding, or frequently visited by birders.

Rare birds Catalonia

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At first glance, it is very apparent that, even though we are dealing with a quite heterogeneous group of species, rare bird records are not evenly distributed over the country. This is due both to natural causes and to an irregular coverage of the territory by birders. The first thing that one may notice is that most rare birds are found in coastal areas or just a few km inland. There are well known reasons for this (migratory routes, migrant influxes driven by winds…). Also very noticeable are three denser blocks, sketching the three major wetlands, which happen to be coastal (Aiguamolls de l’Empordà, Delta del Llobregat and Delta de l’Ebre). But among these, note how Barcelona city and immediate surroundings hold a remarkable number of sightings. This indicates a more constant effort made by birders in those areas, which are closer to many people’s homes and hence, are more easily reached on a regular basis. Same thing happens near Tarragona on a smaller scale, where admittedly, the large harbour also favours the possibility of some rare birds, mostly gulls (but note that one of the “stars” of the area, the rare/scarce but regular Herring Gull, is not treated here).

Besides the importance of coastal areas, it seems quite clear that most records are found in the lowlands. Catalonia is a mountainous country, and its mountains are largely covered by forests, but there is a limited number of rare bird species which favour these habitats. This is not to say that mountains and forests aren’t important for birds; they are, but they tend to live in lower densities. Even when rare birds may be present, finding them proves much more difficult, as the suitable territory is very large, compared to the localized and smaller scale wetlands. Besides, while looking for rare forest birds on small patches of woodland next to wetlands or coasts is often successful, waders or ducks are usually less versatile when it comes to choosing stopover sites.

To be noted is the fact that a good part of the rare bird species are “water birds” (tied one way or another to marshes, rivers, seas, and other bodies of water). Specifically, 68 out of 118. That’s more than half. And then, some others, while not directly linked to water, are more likely to be found in flat open country than on hills and woodland.

A heatmap representation gives us a better idea of the areas that hold a low (light blue), high (dark blue) or highest concentration of records (red). Red areas include the three blocks mentioned earlier, but we see how other secondary areas also achieve high numbers of rare birds: the inland lake of Ivars, Tarragona harbour, Girona town immediate surroundings, Baix Ter marshes, Solius rubbish dump (mainly gulls), Gironès/La Selva plains and Barcelona area. On the other hand, we see large gaps on the map where rare birds have never been found.

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Let’s see some zoomed-in images. The largest concentration of records is found in the Empordà region. Here, a very rich and diverse combination of habitats produces top conditions for birding in general, and also for rare birds to be found. But even here, note how a limited number of relatively small areas hold most of the records. These are well known locations that enjoy a deserved good reputation among birders, and therefore are constantly visited and surveyed, while surroundings receive, understandably, less attention.

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Same thing happens in the Llobregat Delta area, where the airport and expansive urban development has left only a few remaining good spots for birds which attract both common and rare species and, with them, the majority of birders.

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Interestingly, Barcelona city has a remarkable number of records (especially taking into account that this is an urban area), most of them on a few remaining spots with natural or seminatural vegetation which, surrounded by the “concrete jungle”, become a sort of “magnets” for migrant birds, and sometimes for rare ones (Montjuïc, Besòs river, Parc de la Ciutadella). This, combined with the situation near the coast and the larger number of observers, explains the relatively high number of records. By comparison, the much more extensive and largely forested Collserola mountains, just west of the city, hold a rather low number of records.

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Of course, the Ebro Delta region holds a large number of rarities found. Note though, how the intensively cultivated rice fields are not that productive when it comes to rare bird findings, compared to the remaining natural lagoons and bays. Large protected areas (Punta de la Banya, Illa de Buda) are out of reach for most people, as they are not open to the public, so the number of records there is not representative of their true –and extraordinary– ornithological interest.

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Looking inland to Lleida plains (a huge and mostly agricultural region), we see that the central area (darker) has a relatively lower number of records (it is intensively cultivated and irrigated), while most rare birds findings are concentrated on the Ivars lake and surrounding pseudo-steppe habitats (lighter), as well as some artificial freshwater ponds.

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The image of Girona town area shows a high number of records on the river Ter very close to the town centre, with fewer or none as we move upstream or downstream. While the habitat is pretty much the same, and one might think that the chances of finding a rare bird are more or less equal, it is clear that proximity to people’s home is an important factor; the frequent visits to “local patches” pay off.

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To sum up, it is clear that lowlands, coastal areas and wetlands are the winners when it comes to rare bird hotspots. But other factors play also an important role in highlighting some areas over time; man-made places where large concentrations of birds occur (harbours, rubbish-dumps, artificial ponds), areas that act as migrant “magnets” (island effect) and, especially, a more frequent and dedicated “coverage” by observers in some areas than others.

A deeper understanding of every species biology is the key to predict, to some extent, where and when rare birds might appear. But then again, complex systems (such as bird populations), which are linked to other complex and highly variable systems (such as the weather, climate, food availability, individual behaviour, etc.) will always show a certain amount of randomness. So, when out birding, keep checking every bird and, just in case, expect the unexpected.

Considered species:
Horned Grebe
Red-necked Grebe
Cory’s Shearwater
Great Shearwater
Sooty Shearwater
Red-footed Booby
White Pelican
Pygmy Cormorant
Western Reef Heron
Lesser Flamingo
Bewick’s Swan
Whooper Swan
Greater White-fronted Goose
Bean Goose
Pink-footed Goose
Brant Goose
Ruddy Shelduck
Marbled Duck
Green-winged Teal
Blue-winged Teal
Ring-necked Duck
Greater Scaup
Lesser Scaup
Common Eider
Long-tailed Duck
Common Goldeneye
Common Merganser
White-headed Duck
White-backed Vulture
White-tailed Eagle
Spanish Imperial Eagle
Greater Spotted Eagle
Lesser Spotted Eagle
Pallid Harrier
Rough-legged Buzzard
Long-legged Buzzard
Lanner Falcon
Corn Crake
Baillon’s Crake
Allen’s Gallinule
Cream-colored Courser
American Golden Plover
Pacific Golden Plover
Sociable Lapwing
Purple Sandpiper
Broad-billed Sandpiper
Baird’s Sandpiper
White-rumped Sandpiper
Pectoral Sandpiper
Buff-breasted Sandpiper
Spotted Sandpiper
Terek Sandpiper
Lesser Yellowlegs
Great Snipe
Red Phalarope
Red-necked Phalarope
Wilson’s Phalarope
Long-billed Dowitcher
Long-tailed Skua
Bonaparte’s Gull
Caspian Gull
Great Black-backed Gull
Laughing Gull
Ring-billed Gull
Iceland Gull
Franklin’s Gull
Sabine’s Gull
Roseate Tern
Lesser Crested Tern
Royal Tern
Common Guillemot
Blue-cheeked Bee-eater
Shore Lark
Richard’s Pipit
Olive-backed Pipit
Rock Pipit
Citrine Wagtail
Bohemian Waxwing
Rufous-tailed Scrub Robin
Red-flanked Bluetail
Moussier’s Redstart
Desert Wheatear
Isabelline Wheatear
Siberian Stonechat
Naumann’s Thrush
Lesser Whitethroat
Marmora’s Warbler
Moltoni’s Warbler
Aquatic Warbler
Paddyfield Warbler
Blyth’s Reed Warbler
Western Olivaceous Warbler
Eastern Bonelli’s Warbler
Iberian Chiffchaff
Dusky Warbler
Hume’s Leaf Warbler
Yellow-browed Warbler
Pallas’s Leaf Warbler
Collared Flycatcher
Atlas Pied Flycatcher
Red-breasted Flycatcher
Isabelline Shrike
Steppe Grey Shrike
Brown Shrike
Hooded Crow
Rosy Starling
Spanish Sparrow
Common Redpoll
Common Rosefinch
Trumpeter Finch
Lapland Bunting
Snow Bunting
Little Bunting
Rustic Bunting
Pine Bunting
Black-headed Bunting

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