Nearctic vagrants on Corvo, Azores, in 2005-17

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297
The best hot spot in the Western Palearctic (WP)
for Nearctic vagrants is without doubt Corvo,
Azores (gure 1). Every autumn, many European
birders keep an eye on this small island in the
Atlantic, waiting for news about new discoveries
(cf Alfrey 2005, 2006ab, 2008ab, Alfrey et al 2010,
Jones & Legrand 2013, Monticelli et al 2015,
2018). For an introduction to the island, its avi-
fauna, birding locations and logistics, and analyses
of its vagrants in 2005-09, see a previous paper in
Dutch Birding (Alfrey et al 2010; cf gure 1). In
this paper, we limit ourselves mainly to Nearctic
vagrants recorded in 2010-17, and we make com-
parisons with earlier years (cf Monticelli et al
2015).
Vagrancy patterns
Species richness and composition
In 2005-17, 105 Nearctic species have been re-
corded on Corvo, of which 64 concerned land-
birds (appendix 1, gure 2). The number of va-
grants (both waterbirds and landbirds) found each
autumn varied quite a lot from year to year (gure
3). The most productive year was 2017, with 155
individuals of 48 species. Other good years, par-
ticularly for landbirds, were 2012 and 2015, and
poor years were 2014 and 2016. Productive years
for landbirds were also productive for waterbirds,
although 2007 proved to be an exception with an
above-average showing of waterbirds in a poor
Nearctic vagrants on Corvo, Azores,
in 2005-17
Peter Alfrey, David Monticelli, Vincent Legrand & Corvo Birders
FIGURE 1 Birding site locations on Corvo, Azores (© Vincent Legrand & Daniele Occhiato)
[Dutch Birding 40: 297-317, 2018]

298
Nearctic vagrants on Corvo, Azores, in 2005-17
landbird year. Interestingly, since 2010 there have
been more landbird than waterbird species each
year, which contrasts with the 2005-09 period
when more waterbird than landbird species were
tallied in some years (eg, in 2006-07; gure 3). The
annual proportion of waterbirds and landbirds
varied; for instance, 2015 was indeed a good year
for vagrant landbirds but it was also one of the
least productive years for waterbirds, whereas
2012 and 2017 were good for both (gure 3). In
fact, in 2012, phenomenal numbers of waders
were found (appendix 1); below, we look at some
of the factors which may be attributed to this.
After 13 years of getting data for Corvo, interest-
ing vagrancy patterns start to emerge. For instance,
it becomes clear that some vagrant landbird spe-
cies are encountered with some regularity. The
only one that has been recorded in every autumn
since 2005 is Red-eyed Vireo Vireo olivaceus.
Common Yellowthroat Geothlypis trichas turned
up in all but one year. After these two, the most
frequently recorded species were, eg, Yellow-
billed Cuckoo Coccyzus americanus, Philadelphia
Vireo V philadelphicus, American Cliff Swallow
Petro chelidon pyrrhonota, Grey-cheeked Thrush
Catha rus minimus, American Buff-bellied Pipit
Anthus rubescens rubescens, Scarlet Tanager Piran-
ga olivacea, Rose-breasted Grosbeak Pheuc ticus
ludovicianus, Indigo Bunting Passerina cyanea,
Bobolink Dolichonyx oryzi vorus, Black-and-white
22 25 26
32
39
44 47
52 56 57
61 61 64
0
10
20
30
40
50
60
70
2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
7
10
16
13 14
18
15 16
12
8 8
12
19
21
9
7 14
24
20
22
26
24
16
27
14
29
0
5
10
15
20
25
30
35
40
45
50
2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
Landbirds
Non-landbirds
FIGURE 2 Cumulative number of Nearctic landbird
species on Corvo, Azores, in 2005-17
FIGURE 3 Annual number of Nearctic species on Corvo, Azores, in 2005-17

299
Nearctic vagrants on Corvo, Azores, in 2005-17
393 Cedar Waxwing / Cederpestvogel Bombycilla cedrorum, rst-year, Middle Fields, Corvo, Azores, 12 October
2013 (Vincent Legrand) 394 American Bittern / Noord-Amerikaanse Roerdomp Botaurus lentiginosus, Fojo, Corvo,
Azores, 29 October 2013 (Richard Bonser) 395 Yellow-crowned Night Heron / Geelkruinkwak Nyctanassa violacea,
rst-year, Lower Fields, Corvo, Azores, 1 October 2011 (Vincent Legrand)

300
Nearctic vagrants on Corvo, Azores, in 2005-17
0 1 2 3 4 5 6 7 8 9 10 11 12 13
Red-eyed Vireo
Common Yellowthroat
Indigo Bunting
Rose-breasted Grosbeak
American Buff-bellied Pipit
Scarlet Tanager
Bobolink
Grey-cheeked Thrush
Philadelphia Vireo
Northern Parula
Yellow-billed Cuckoo
American Cliff Swallow
Black-and-white Warbler
Chimney Swift
Blackpoll Warbler
Black-throated Blue Warbler
Black-throated Green Warbler
Myrtle Warbler
Ovenbird
Common Nighthawk
Tree Swallow
American Barn Swallow
Swainson’s Thrush
American Robin
Tennessee Warbler
Northern Waterthrush
American Redstart
American Yellow Warbler
Baltimore Oriole
Northern Harrier
Mourning Dove
Cedar Waxwing
White-eyed Vireo
Yellow-throated Vireo
Hooded Warbler
Canada Warbler
Summer Tanager
White-crowned Sparrow
Lincoln’s Sparrow
Dickcissel
Rough-legged Hawk
Northern Flicker
Hermit Thrush
Magnolia Warbler
Blue-winged Warbler
American Tundra Peregrine Falcon
Belted Kingfisher
Black-billed Cuckoo
Yellow-bellied Sapsucker
American Northern Shrike
Purple Martin
Ruby-crowned Kinglet
Eastern Wood-Pewee
Wood Thrush
Veery
Blackburnian Warbler
Bay-breasted Warbler
Chestnut-sided Warbler
Prairie Warbler
Golden-winged Warbler
‘Brewster’s Warbler’
Yellow-throated Warbler
White-throated Sparrow
Savannah Sparrow
Warbler Mniotilta varia and Northern Parula Setophaga americana, all
seen in over 50% of autumns since 2005 (gure 4). On the other
hand, no less than 18 American landbird species and one subspecies
(American Peregrine Falcon Falco peregrinus tundrius) have only
been recorded in one autumn, indicating that the chance to nd a
new species each autumn is high (gure 4).
Comparison with western Europe
It is interesting to compare the Nearctic landbirds’ vagrancy pat-
tern and species composition of Corvo (ie, mid-Atlantic) with those
elsewhere in western Europe, particularly the British Isles. The
most striking difference is that Myrtle Warbler S coronata has only
been recorded ve out of 13 autumns on Corvo, in a frequency
similar to that of extreme vagrants as Black-throated Blue S caer-
ulescens and Black-throated Green Warbler S virens (although
the cumulative number of Myrtle Warblers is larger with more
than one in some autumns; cf Mitchell 2017). Another striking
difference concerns the regular occurrence of Indigo Bunting
on Corvo, with a total of 39 records (50+ individuals) in 11
autumns, while there have only been two in the British Isles
(Mitchell 2017). Philadelphia Vireo is another species re-
corded in a higher frequency on Corvo than in the rest of
Europe, whereas Swainson’s Thrush C ustulatus is an exam-
ple of a species recorded less frequently on Corvo than
further north-east in western Europe.
In gure 5, the frequency of American waterbird
records in autumn on Corvo is shown. This pattern is
similar to that elsewhere in western Europe with species
such as American Golden Plover Pluvialis dominica,
White-rumped Sandpiper Calidris fuscicollis, Pectoral
Sandpiper C melanotos, Lesser Yellowlegs Tringa a-
vipes and, to a lesser extent, Blue-winged Teal Anas
discors being amongst the most often seen. American
Black Ducks A rubripes or its hybrids in gure 5
concern a resident population (appendix 1). There
are noticeable differences with respect to numbers
of Semipalmated Plover Charadrius semipalma-
tus and Wilson’s Snipe Gallinago delicata, as
both are more regular as vagrants in the
Azores than further north-east in western
Europe. Great Blue Heron Ardea herodias
is another example of a Nearctic vagrant
occurring more frequently in the
Azores whereas, in proportion,
Baird’s Sandpiper C bairdii and
Long-billed Dowitcher Lim-
nodromus scolo paceus
are recorded less fre-
quent ly here (cf
Mitchell 2017).
FIGURE 4 Number of record years of Nearctic landbirds on Corvo, Azores, in 2005-17

301
Nearctic vagrants on Corvo, Azores, in 2005-17
0 1 2 3 4 5 6 7 8 9 10 11 12 13
White-rumped Sandpiper
American Black Duck & hybrids
Pectoral Sandpiper
Blue-winged Teal
Wilson’s Snipe
American Golden Plover
Lesser Yellowlegs
Semipalmated Sandpiper
Spotted Sandpiper
Great Blue Heron
Semipalmated Plover
Buff-breasted Sandpiper
Ring-necked Duck
Green-winged Teal
American Great Egret
Greater Yellowlegs
Laughing Gull
Ring-billed Gull
American Wigeon
Least Sandpiper
Upland Sandpiper
Hudsonian Whimbrel
Solitary Sandpiper
American Herring Gull
Todd’s Canada Goose
Double-crested Cormorant
American Bittern
Killdeer
Long-billed Dowitcher
Short-billed Dowitcher
Surf Scoter
Pied-billed Grebe
Sora
Yellow-crowned Night Heron
Baird’s Sandpiper
Wilson’s Phalarope
Bonaparte’s Gull
Forster’s Tern
These variations in occurrence are presumably related to varying
migration strategies in different species. Factors like timing, routes
and susceptibility to weather-related events may determine whether
species tend to show an emphasis on vagrancy to the mid-Atlantic
(Corvo) or all the way transatlantic (Britain and the rest of Europe).
The southerly location of the Azores may favour the arrival of mi-
gratory species with a more southerly distribution in North America
such as, eg, Yellow-crowned Night Heron Nyctanassa violacea,
White-eyed Vireo V griseus and Yellow-throated Warbler S domi-
nica.
Waders and ducks (waterbirds) are known to migrate at consid-
erable height, so the effects of jet streams are often considered to
be a more determining factor for them than for smaller landbirds
that may migrate at lower elevations and are more inuenced
by pressure systems (Elkins 1995, Newton 2008). Besides,
waterbirds like ducks, gulls, terns and sometimes waders can of
course rest on the sea surface and, consequently, adopt a more
versatile migration strategy – passerines (landbirds) have less
options, so their migration strategies are presumably more
constrained (Newton 2008). However, a new study shows
that some species of landbirds also migrate at heights of
4000 m asl (Sjöberg et al 2018).
Timing of arrival and discovery
The peak time for Nearctic vagrants on Corvo is late
September to early November. Within this period, it is dif-
cult to decide what are actually the best weeks.
Observers’ effort is a major factor and both late Sep-
tember and early November have been poorly covered
as most birders have focused on mid-October. Most
vagrants have been discovered between 6 and 26
October and the weeks with the highest results are
13-19 and, especially, 20-26 October (gure 6-7).
The actual peak arrival times vary from year to
year (gure 6) and are likely to be inuenced
by weather conditions. In 2015, few vagrants
were recorded before 17 October after a
period of prolonged easterly winds. Until
that date, it had been such a ‘bad’ au-
tumn that some birders left Corvo ear-
lier than planned to y home while
others cancelled their visits (see
also below). However, after
17 October, it became one of
the best autumns on record
(gure 6).
FIGURE 5 Number of record years of Nearctic waterbirds on Corvo, Azores, in 2005-17

302
Nearctic vagrants on Corvo, Azores, in 2005-17
396 American Tundra Peregrine Falcon / Amerikaanse Toendraslechtvalk Falco peregrinus tundrius, juvenile, High
Fields, Corvo, Azores, 3 October 2013 (David Monticelli) 397 Rough-legged Hawk / Amerikaanse Ruigpootbuizerd
Buteo lagopus sanctijohannis, Pico, Corvo, Azores, 14 October 2013 (Vincent Legrand) 398 Todd’s Canada Goose /
Todds Canadese Gans Branta canadensis interior, subadult, Lower Fields, Corvo, Azores, 16 October 2012 (Vincent
Legrand) 399 American Herring Gull / Amerikaanse Zilvermeeuw Larus smithsonianus, subadult, Lower Fields,
Corvo, Azores, 22 October 2012 (Vincent Legrand) 400 Surf Scoter / Brilzee-eend Melanitta perspicillata, juvenile,
New Harbour, Corvo, Azores, 17 October 2017 (Daniele Occhiato) 401 Solitary Sandpiper / Amerikaanse Bosruiter
Tringa solitaria, Mountain Reservoir, Corvo, Azores, 8 October 2012 (Vincent Legrand)

303
Nearctic vagrants on Corvo, Azores, in 2005-17
402 Bay-breasted Warbler / Kastanjezanger Setophaga castanea, rst-year male, Lighthouse Valley, Corvo, Azores,
22 October 2017 (David Monticelli) 403 American Northern Shrike / Amerikaanse Noordelijke Klapekster Lanius
borealis borealis, rst-year, Lighthouse Valley, Corvo, Azores, 18 October 2014 (David Monticelli) 404 Blue-winged
Warbler / Blauwvleugelzanger Vermivora cyanoptera, rst-year male, Ribeira da Ponte, Corvo, Azores, 6 October
2011 (Vincent Legrand) 405 Northern Flicker / Gouden Grondspecht Colaptes auratus, male, High Fields, Corvo,
Azores, 15 October 2010 (Vincent Legrand) 406 Blackburnian Warbler / Sparrenzanger Setophaga fusca, rst-year
male, Tennessee Valley, Corvo, Azores, 15 October 2017 (Vincent Legrand) 407 Golden-winged x Blue-winged
Warbler (‘Brewster’s Warbler’) / Geelvleugelzanger x Blauwvleugelzanger Vermivora chrysoptera x cyanoptera, rst-
year female, Ribeira da Ponte, Corvo, Azores, 21 October 2015 (Vincent Legrand)

304
Nearctic vagrants on Corvo, Azores, in 2005-17
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017
22-28 Sep
29 Sep-5 Oct
6-12 Oct
13-19 Oct
20-26 Oct
27 Oct-2 Nov
3-9 Nov
Number of Nearctic landbird species
0
10
20
30
40
50
60
70
22-28
Sep
29 Sep-
5 Oct
6-12
Oct
13-19
Oct
20-26
Oct
27 Oct-
2 Nov
3-9
Nov
10-16
Nov
17-23
Nov
24-30
Nov
2006
2007
2008
2009 2010
2011
2012
2013
2014
2015
2016
2017
2005
0
5
10
15
20
25
30
35
40
45
50
10 15 20 25 30 35 40 45 50
Number of Nearctic species
Number of cyclonic events in North Atlantic
FIGURE 6 Discovery dates of Nearctic landbirds on Corvo, Azores, in 2005-17
FIGURE 7 Number of Nearctic landbirds discovered in
each week from late September to late November on
Corvo, Azores, in 2005-17 (all years pooled)
FIGURE 8 Number of Nearctic vagrants on Corvo, Azores,
in relation to number of cyclonic events in North Atlantic
in 2005-17. Annual totals of cyclonic events (tropical
storms and hurricanes) observed during hurricane
season (1 June-30 November) in North Atlantic ex-
tracted from https://en.wikipedia.org/wiki/Accumulated_
cyclone_energy.

305
Nearctic vagrants on Corvo, Azores, in 2005-17
Weather, observer effort and vagrancy analysis
Overall trend: 2005-17
The causes of vagrancy are complex (Gilroy & Lees
2003, Alfrey 2008ab). They can relate to genetic
pre-determination, trauma and anthropogenic, be-
havioral or environmental factors. Birders on
Corvo use the internet (eBird) to check the ow of
migration along the eastern coasts of North
America as a direct correlation is expected be-
tween the numbers of birds on the move and the
potential for vagrancy. More work should be done
to investigate possible correlations between va-
grancy occurrences on Corvo in the past and an-
nual uctuations in numbers in North America.
Such correlations, if established, could be useful in
short-term predictions.
However, the most important variable for va-
grancy is weather (gure 8). Weather conditions
leading to mid-Atlantic vagrancy appear to be dif-
ferent from those leading to transatlantic vagrancy
(Elkins 1995). Fast-moving depressions are less of a
factor for the Azores (ie, mid-Atlantic) and more so
is the presence of mid-latitude depressions (not
necessarily deep or fast moving) or simply a weak
to moderate westerly airow. The association of
tropical cyclones is not a pre-requisite (but there is
a correlation) for mid-Atlantic vagrancy: 2009,
2013 and 2015 were all good years on Corvo
while Atlantic cyclonic activity was low during
these years (gure 8). Tropical cyclones are usually
contributory factors but, importantly, the cyclones
need to disrupt migrations along the eastern shores
of the USA while coinciding with a westerly mid-
latitude air ow to deect disoriented birds into
the Atlantic. A hurricane that moves across the
Atlantic from western Africa and collides with the
Florida area, then moves inland, is unlikely to ex-
press any impacts on the Azores, especially in the
absence of a westerly airow towards the islands.
Other factors include observer coverage and the
quality of that coverage. It is clear that 2005 is an
exception (gure 8). This can be explained by the
fact that, in 2005, PA was the sole observer on
Corvo being the rst to discover the island’s full
potential for Nearctic vagrants. PA ‘only’ recorded
28 Nearctic species but it is possible to extrapolate
from the best t line that there were actually be-
tween 40 and 45 Nearctic species on the island
that year, ie, that PA only found 66% of the species
present.
Of course, it is not so that each additional ob-
server will double the discovery rate, ie, the fact
that one observer can nd 66% of the species does
not mean that two will nd 100%. It would be in-
teresting to estimate the optimum number of ob-
servers required to maximize the chance to dis-
cover every species present. It is not easy though,
as some species are more difcult to discover than
others, and not everyone has the same skill to nd
rarities. For instance, of 80 birders present on
Corvo in autumn 2017, some would not have dis-
covered any of the 148 Nearctic birds that year.
Interestingly, anthropogenic factors are con-
sidered so important on Corvo that there is a daily
routine of checking the main maritime routes and
the density of ships passing nearby. A large ship
passing by is considered just as encouraging as
some promising weather systems; see, eg, Alfrey
(2008b) for more information on ship-assisted ar-
rivals by birds.
Annual trend: 2012, 2015 and 2017 – the fat years
The years 2012, 2015 and 2017 were the ‘best’ for
American landbird vagrants on Corvo. For all
Nearctic vagrants combined (waterbirds and land-
birds) 2012 and 2017 were the best, but 2009,
2010, 2011 and 2013 were good years too, with
more Nearctic waterbirds and landbirds combined
than in 2015. However, most birders come to
Corvo for landbirds, so here we pay extra attention
to the best landbird years.
2012
In terms of quality birds, 2012 was outstanding with
Wood Thrush Hylocichla mustelina, Dickcissel
Spiza americana, Lincoln’s Sparrow Melospiza lin-
colnii, Golden-winged Warbler Vermivora chryso-
ptera, Magnolia Warbler S magnolia and Prairie
Warbler S discolor (rst for the WP) – amongst a
total of 42 Nearctic species (26 landbird species).
There was also a major wader invasion in the Azores
(appendix 1) so, presumably, there were favourable
jet streams in addition to favourable lower altitude
weather systems. Following a rather slow start in
the rst days of October the pace started to pick up
from 6 October onwards with a pressure system
pulling in north-westerly winds from the western
Atlantic. Though not particularly deep or fast mov-
ing (gure 9), the system kept on producing birds.
There was a further inux of Nearctic vagrants on
16 October. An examination of the synoptic charts
for that date shows no cyclonic activity at all, in fact
there was a high-pressure system producing a light
westerly breeze on its northern ank (gure 10).
2015
2015 was an interesting year. As mentioned earlier,
it was so quiet in the rst two weeks of October that
birders started to cancel their trips, while others left

306
Nearctic vagrants on Corvo, Azores, in 2005-17
408 Hermit Thrush / Heremietlijster Catharus guttatus, rst-year, Mountain Reservoir, Corvo, Azores,
24 October 2015 (Daniele Occhiato)
409 Swainson’s Thrush / Dwerglijster Catharus ustulatus, rst-year, Lighthouse Valley, Corvo, Azores,
20 October 2015 (Daniele Occhiato)

307
Nearctic vagrants on Corvo, Azores, in 2005-17
410 Northern Waterthrush / Noordse Waterlijster Parkesia noveboracensis, rst-year, Cape Verdian Fields, Corvo,
Azores, 10 October 2012 (Vincent Legrand)
411 American Robin / Roodborstlijster Turdus migratorius, rst-year, High Fields, Corvo, Azores, 18 October 2012
(David Monticelli)

308
the island. Figure 11 shows the synoptic chart for
early October, when a high pressure system cen-
tred well north of the Azores was bringing in a light
northerly wind. However, on 17 October, there
was a slight shift with a north-westerly component
(gure 12) and, completely unexpectedly, there
was a major fall! By the end of the day, a Grey-
cheeked Thrush, an American Robin Turdus migra-
torius, three American Buff-bellied Pipits, two
Philadel phia Vireos, seven Red-eyed Vireos, three
Scarlet Tanagers, four Rose-breasted Gros beaks,
two Indi go Buntings and an Ovenbird Seiurus auro-
capilla were discovered (mainly around the vil-
lage), but the star of the day was a stunning Blue-
winged Warbler V cyanoptera. In the aftermath of
this event more birds were discovered.
2017
2017 was by far the most productive Corvo season
so far, with 48 Nearctic species and 155 individu-
als (gure 3). The birds were spread out across the
season (gure 13) including an atypical peak in
late September (only in 2008 there was a similar
interesting early period). The late September in-
ux coincided with early autumn cyclonic activity
(gure 14). This was followed by a ‘dead spell’ in
perfect anticyclonic weather with no wind and
clear, starlit nights (gure 15). Then, the vagrants
started to appear again with the establishment of a
westerly airow from around 14 October onwards
(gure 16). When hurricane Ophelia appeared on
the weather maps around 15 October, it was not-
ed as an unusual feature being the most easterly
Nearctic vagrants on Corvo, Azores, in 2005-17
FIGURE 9 Synoptic chart for 7 October 2012 (Met Ofce). Westerly airows were generated fairly consistently
throughout October 2012. In this case, a relatively weak low pressure system was involved.
FIGURE 10 Synoptic chart for 16 October 2012 coinciding with an inux of Nearctic vagrants (Met Ofce). This is a
case of high pressure generated mid-Atlantic vagrancy with a westerly air breeze on the northern ank of a high pres-
sure. The often quoted causes of European trans-Atlantic vagrancy, fast moving depressions and hurricanes, play a
relatively minor role in vagrant occurrence on Corvo (see text for more details).
FIGURE 11 General situation prior to 17 October 2015 (Met Ofce). The Azores high was well established through
much of early October 2015.
FIGURE 12 Synoptic chart for 17 October 2015, the day of a Nearctic fall (Met Ofce). Just a slight re-orientation of
a system with a shift from northerly to north-westerly winds produced a remarkable effect.
In all charts, position of Azores indicated by .

309
Nearctic vagrants on Corvo, Azores, in 2005-17
formed hurricane in the North Atlantic since 1980.
The forecast was for the hurricane to track north
towards Britain. At no point would it get anywhere
near the USA. Therefore some birders were not
expecting it to have much impact on Nearctic
vagrancy. However, the hurricane would act as a
mas sive vacuum, dragging a westerly airow
across the Atlantic – and with it came the vagrants
(gure 17). Weather conditions in the immediate
aftermath on Corvo were characterised by strong
winds and also an elusive Blackburnian Warbler
S fusca and a Yellow-throated Vireo V avifrons
had distracted most bird nders. By 20 October,
when the wind had calmed and most birders had
seen the Yellow-throated and Blackburnian, sig-
nicant nds were made. This peak on 20 October
(gure 13), is an example of an observer-generat-
ed peak as a result of observer effort and (good)
nding conditions rather than directly related to
weath er. The log for that day (http://birding
corvo2013.blogspot.co.uk) had four Yellow-billed
Cuckoos, eight Red-eyed Vireos, single Grey-
cheeked Thrush, Scarlet Tanager, Dickcissel, Rose-
breasted Grosbeak and Indigo Bunting, three
Blackpoll Warblers S striata, single Ovenbird,
Black-and-white Warbler, Tennessee Warbler
Oreo thlypis peregrina, Hooded Warbler S citrina
and Black-throated Green Warbler, and three
Common Yellowthroats – an amazing day!
Annual trend: 2014 and 2016 – the lean years
Two years ‘stand out’ as very poor on Corvo – 2014
and 2016. They offer more insight into the condi-
tions that do not lead to Nearctic vagrancy. 2016
will long be remembered as the nightmare year,
sandwiched in between the two epic years of 2015
and 2017. What made it worse was that, in Britain
and continental Europe, the autumn of 2016 prob-
ably was the best for eastern vagrants ever, so bird-
ers ‘stuck’ on Corvo could only watch with envy.
The same climatic conditions that generated a ma-
jor eastern vagrant inux, including that of Siberian
Accentors Prunella montanella in north-western
Europe (Stoddart 2018), was also responsible for
the poor Corvo autumn. A major high pressure sys-
tem centred over Scandinavia was dragging in
easterly winds from across the whole of Eurasia
(gure 18). This had the effect of pushing the
Azores high pressure north-west with resulting
calm to weak easterly wind conditions. The system
was very stable and the conditions were main-
tained through much of October. As expected, it
was also winds prevailing from the north and east
that caused a poor year in 2014. However, even in
0
5
10
15
20
25
30
Nearctic landbirds
Nearctic waterbirds
23 24 25 26 27 28 29 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 2728 29 30 31
September October
FIGURE 13 Daily totals of Nearctic species on Corvo, Azores, in autumn 2017

310
Nearctic vagrants on Corvo, Azores, in 2005-17
FIGURE 14 Synoptic chart for 23 September 2017 (Met
Ofce). A major depression swept past the Azores with
a westerly airow generated to the Azores from the east-
ern seaboard of the USA. Peter Stronach and company
found Black-throated Blue Warbler Setophaga caerules-
cens, Black-throated Green Warbler S virens and Canada
Warbler Cardellina canadensis during this early autumn
trip.
FIGURE 15 Synoptic chart for 9 October 2017 (Met
Ofce). Completely calm conditions over Corvo – a rar-
ity seeker’s nightmare.
FIGURE 16 Synoptic chart for 14 October 2017 (Met
Ofce). A high pressure is involved again here in gener-
ating a westerly airow to the Azores and subsequent
arrival of vagrants.
FIGURE 17 Synoptic chart for 16 October 2017 (Met Ofce). Hurricane Ophelia moves north from the Eastern
Atlantic and drags a conveyor belt of westerly winds across the Atlantic producing dramatic vagrancy effects on
Corvo.
FIGURE 18 Synoptic chart for 15 October 2016 (Met Ofce). A high-pressure cell centred over Scandinavia creates a
large conveyor of easterly air across Eurasia generating an unprecedented invasion of Siberian vagrants. The same
system was responsible for a blocking high over the Western Atlantic, resulting in a very low Nearctic vagrant activ-
ity on Corvo.
In all charts, position of Azores indicated by .

311
Nearctic vagrants on Corvo, Azores, in 2005-17
412 Ruby-crowned Kinglet / Roodkroonhaan Regulus calendula, rst-winter, Lighthouse Valley, Corvo, Azores,
1 November 2015 (David Monticelli) 413 Dickcissel / Dickcissel Spiza americana, rst-year male, Middle Fields,
Corvo, Azores, 16 October 2012 (David Monticelli) 414 Veery / Veery Catharus fuscescens, rst-year, Fojo, Corvo,
Azores, 15 October 2015 (Vincent Legrand) 415 White-throated Sparrow / Witkeelgors Zonotrichia albicollis, rst-
year male, elds between Mountain Reservoir and Caldeirao, Corvo, Azores, 16 October 2013 (Kris De Rouck)
these poor years there were still 14 Nearctic land-
bird vagrants recorded in 2016 and 16 in 2014.
So, visiting birders, especially those who visit
Corvo for the rst time, always stand a good chance
of seeing Nearctic vagrants ‘new for their lists’
even in the worst years.
Conclusion
Since 2010, there have been four ‘rsts‘ for the WP
discovered on Corvo: Eastern Wood Pewee
Contopus virens (two records), Lincoln’s Sparrow
(three records), Yellow-throated Warbler and Prai-
rie Warbler (Monticelli et al 2018). Additionally,
there have been other signicant WP discoveries
including, eg, Yellow-crowned Night Heron,
Rough-legged Hawk Buteo lagopus sanctijohannis
(four records), Northern Flicker Colaptes auratus
(two records), Purple Martin Progne subis, Ruby-
crowned Kinglet Regulus calendula, Wood Thrush,
Dickcissel, Golden-winged Warbler, Blue-winged
Warbler (two records), Bay-breasted Warbler S cas-
tanea and Blackburnian Warbler. Perhaps most
signicant of all was the rst WP record of a hybrid
of Golden-winged x Blue-winged Warbler Vermi-
vora chrysoptera x cyano ptera (so-called ‘Brewster’s
Warbler’).
Figure 2 shows that there has been no let-up in
the number of new species recorded year on year.
One can only speculate as to what the next new
species for Corvo will be. As mentioned in Alfrey
et al (2010) the next important development in
‘Corvo birding’ is the expansion onto the neigh-

312
416 Golden-winged Warbler / Geelvleugelzanger Vermivora chrysoptera, rst-year male, Lighthouse Valley, Corvo,
Azores, 13 October 2012 (Vincent Legrand)
417 Magnolia Warbler / Magnoliazanger Setophaga magnolia, rst-year male, Ribeira da Ponte, Corvo, Azores,
19 October 2017 (Vincent Legrand)
Nearctic vagrants on Corvo, Azores, in 2005-17

313
bouring island of Flores. This expansion has taken
a step back as in recent years the facilities for bird-
ers on Corvo increased. However, even the newly
available housing is getting all lled. If there are
groups or individuals who have not been able to
book a place on Corvo, please contact PA to ar-
range accommodation and boat transport between
Flores and Corvo. Without doubt the story so far, is
not even half the story. When the birding commu-
nity grows both on Corvo and Flores in the coming
years, more and more birds will be found, our un-
derstanding of how vagrancy works will increase,
and we will get a bigger and bigger WP list.
Acknowledgements
Thanks to the people of Corvo for tolerating the c 10%
increase in their population every autumn caused by vis-
iting birders. We are especially grateful to Kathy Rita,
Manuel Rita, the Mayor of Corvo and many others who
transport us around the island, feed us, supply beer and
open up their houses to accommodate us. This paper is
the result of the hard work of all Corvo Birders that have
visited the island during the study period and a tribute to
that birding effort. Particular thanks to Josh Jones for im-
provements to the text of this paper.
Samenvatting
Ne a r c t i s c h e d wa a l g a s t e N o p co r v o , az o r e N , iN 2005-17
Het eiland Corvo, Azoren, is de beste plek in de WP
voor Nearctische dwaalgasten. In de periode 2005-17
zijn er 105 Nearctische soorten vastgesteld, waarvan 64
soorten land vogels. Ieder najaar is verschillend en tot nu
toe was 2017 het productiefste jaar met 155 individuen
van 48 soorten.
Vergeleken met elders in West-Europa zijn soorten als
Dwerg lijster Catharus ustulatus en Mirtezanger Seto-
phaga coronata – wellicht verrassend – relatief schaars
op Corvo, terwijl daarentegen in de rest van West-
Europa zeer zeldzame dwaalgasten als Philadelphia-
vireo Vireo philadelphicus en Indigo gors Passerina cya-
nea op Corvo tot de meest frequent vast gestelde Nearc-
tische zangvogels behoren. Deze variaties hebben ver-
moedelijk te maken met verschillende trekstrategieën en
met de zuidelijke ligging van de Azoren.
Er zijn Nearctische dwaalgasten vastgesteld tussen
eind september en begin november maar de beste peri-
ode is tussen 13 en 26 oktober.
Weerscondities die het voorkomen van Nearctische
dwaalgasten op de Azoren beïnvloeden verschillen van
die bij noordtransatlantische dwaalgasten in West-
Europa. Snel trekkende oceaandepressies spelen een
geringere rol terwijl depressies op lagere breedtegraden
belangrijker zijn. De activiteit van tropische cyclonen
zijn eveneens van belang maar het is vooral de exacte
route van deze cyclonen en de wisselwerking met an-
dere weersystemen die het verschijnen van middenat-
lantische dwaalgasten beïnvloeden.
In de hier besproken periode waren 2012, 2015 en
2017 de beste jaren en de minst productieve jaren
waren 2014 en 2016; deze laatste werden gekenmerkt
door over wegend oostelijke winden.
Online sources
Bird news from the Azores: http://azores.avesdeportugal.
info.
Azores bird database: www.sr-oland.se/azores/index.
html.
Corvo blog: www.birdingcorvo2013.blogspot.co.uk.
Corvo Birders Facebook: www.facebook.com/groups/
1047984388554518.
Azores Nature: www.azoresnature.co.uk.
Azores Choice Travel Agency: www.azoreschoice.com.
References
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Corvo, Azores, in October 2005. Birding World 18:
465-474.
Alfrey, P 2006a. The best locality for American vagrants
in the Western Palearctic: Corvo, Azores, in autumn
2006. Birding World 19: 475-480.
Alfrey, P 2006b. Eye of the storm. Birdwatch 172:
37-40.
Alfrey, P 2008a. The pioneers. Birdwatch 196: 38-42.
Alfrey, P 2008b. Fate or free will. Birdwatch 197: 33-
36.
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Azores 2010. Birding on Corvo, Azores, and Nearctic
vagrants in 2005-09. Dutch Birding 32: 299-315.
Azores Bird Club 2014, 2015. Azores rare and scarce
bird report 2013, 2014. Azores Nature.
Elkins, N 1995. Weather and bird behaviour. London.
Gilroy, J J & Lees, A 2003. Vagrancy theories: are autumn
vagrants really reverse migrants? Br Birds 96:
427-438.
Jones, J & Legrand, V 2013. Autumn 2012 on Corvo,
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Mitchell, D 2017. Birds of Europe, North Africa and the
Middle East. An annotated checklist. Barcelona.
Nearctic vagrants on Corvo, Azores, in 2005-17
418 Wood Thrush / Amerikaanse Boslijster Hylocichla
mustelina, rst-year, Fojo, Corvo, Azores, 9 October
2012 (Vincent Legrand)

314
APPENDIX 1 Number of records (with number of individuals in parentheses) of Nearctic vagrants on Corvo, Azores. Data sourced from
Birding Azores archived data (up to and including 2012), Azores Bird Sightings, Aves dos Açores and the Azores rare and scarce bird
reports 2013 and 2014 (Azores Bird Club 2014, 2015) and 2015, 2016 and 2017 (Azores Bird Club in preparation). Pre-2010 records
are sourced from table 1 in Alfrey et al (2010). Not all records listed have been processed by the Portuguese rarities committee, yet.
Species Pre-2010 2010 2011 2012 2013 2014 2015 2016 2017 Total
number
of records
Todd’s Canada Goose
Branta canadensis interior 1 1 (2) 2
Surf Scoter
Melanitta perspicillata 2 2
Hooded Merganser
Lophodytes cucullatus 1 1
Ring-necked Duck
Aythya collaris 5 1 (2) 4 10
Blue-winged Teal
Anas discors 4 1 1 1 1 1 1 3 (6) 13
American Wigeon
Anas americana 3 1 1 (4) 5
American Black Duck
Anas rubripes 5 (15-27) 1 3 (4) 1 1 1 2 (6) 14
(resident birds including hybrids) (+5 hybrids)
Green-winged Teal
Anas carolinensis 1 1 1 (2) 1 (3) 4
Wood Duck
Aix sponsa 2 2
Pied-billed Grebe
Podilymbus podiceps 1 1
American Mourning Dove
Zenaida macroura 1 1 2 4
Common Nighthawk
Chordeiles minor 3 1 4
Chimney Swift
Chaetura pelagica 3 1 1 1 6
Black-billed Cuckoo
Coccyzus erythropthalmus 1 1
Yellow-billed Cuckoo
Coccyzus americanus 9 2 2 2 (4) 3 (6) 18
Sora
Porzana carolina 1 1
Yellow-crowned Night Heron
Nyctanassa violacea 1 1
American Bittern
Botaurus lentiginosus 1 1 2
Great Blue Heron
Ardea herodias 3 1 1 1 6
American Great Egret
Ardea egretta 2 1 1 4
Nearctic vagrants on Corvo, Azores, in 2005-17
Peter Alfrey, Little Oak, 23 Wood Street, Hackbridge, CR4 4JT Surrey, UK
(littleoakgroup@btinternet.com)
David Monticelli, Parklaan 21, 8430 Middelkerke, Belgium (monticelli.david@gmail.com)
Vincent Legrand, Voskapelstraat 7 box 4, 1933 Sterrebeek, Belgium (legrandtwitch@gmail.com)
Monticelli, D, Jones, J & Legrand, V 2015. New World
vagrants on Corvo, Azores. Macaronesian Birds 1:
28-48.
Monticelli, D, Alfrey, P, Jones, J, Joynt, G, Didner, E,
Dufourny, H, Legrand, V & Crochet, P-A 2018.
Lincoln’s Sparrows, Prairie Warbler, Yellow-throated
Warbler and Eastern Wood Pewees on Corvo, Azores,
in 2010-16. Dutch Birding 40: 285-296.
Newton, I 2008. The migration ecology of birds. Oxford.
Sjöberg, S, Pedersen, L, Malmiga, G, Alerstam, T,
Hansson, B, Hasselquist, D, Thorup, K, Tøttrup, A P,
Andersson, A & Bäckman, J 2018. Barometer logging
reveals new dimensions of individual songbird
migration. J Avian Biol. Website: https://tinyurl.com/
y9fzxunc.
Stoddart, A 2018. Siberian Accentors in Europe in au-
tumn 2016 and the rst British records. Br Birds 111:
69-83.

315
Nearctic vagrants on Corvo, Azores, in 2005-17
Double-crested Cormorant
Phalacrocorax auritus 2-3 1 3-4
American Golden Plover
Pluvialis dominica 3 1 2 3 (4) 1 1 (2) 1 1 (2) 13
Killdeer
Charadrius vociferus 1 2 (3) 3
Semipalmated Plover
Charadrius semipalmatus 10 1 3 (11) 1 15
Upland Sandpiper
Bartramia longicauda 1 1 1 3
Hudsonian Whimbrel
Numenius hudsonicus 4 4
Baird’s Sandpiper
Calidris bairdii 2 2
White-rumped Sandpiper
Calidris fuscicollis 11 4 (12) 3 (7) 3 (55+) 2 (6) 3 (10) 2 (10+) 6 (11) 34
Least Sandpiper
Calidris minutilla 2 2 1 5
Buff-breasted Sandpiper
Calidris subrucollis 1 2 (3) 2 1 1 1 8
Pectoral Sandpiper
Calidris melanotos 12 4 3 2 2 2 1 (3) 2 28
Semipalmated Sandpiper
Calidris pusilla 5 3 (5) 2 (3) 3 (4) 1 (2) 1 (5) 3 18
Wilson’s Phalarope
Phalaropus tricolor 1 1
Spotted Sandpiper
Actitis macularius 5 2 1 1 1 3 13
Solitary Sandpiper
Tringa solitaria 1 1 1 3
Greater Yellowlegs
Tringa melanoleuca 1 2 1 1 5
Lesser Yellowlegs
Tringa avipes 4 2 (3) 2 (3) 1 2 1 1 2 (3) 15
Short-billed Dowitcher
Limnodromus griseus 1 1 2
Long-billed Dowitcher
Limnodromus scolopaceus 1 1 2
Wilson’s Snipe
Gallinago delicata 6 4 1 2 2 1 1 2 19
Bonaparte’s Gull
Chroicocephalus philadelphia 1 1
Laughing Gull
Larus atricilla 3 1 2 6
Ring-billed Gull
Larus delawarensis 1 1 2
American Herring Gull
Larus smithsonianus 1 1 1 3
American Black Tern
Chlidonias niger surinamensis 1 1
Forster’s Tern
Sterna forsteri 1 1
Northern Harrier
Circus hudsonius 1 1 1 3
Rough-legged Hawk
Buteo lagopus sanctijohannis 1 2 3
Belted Kingsher
Megaceryle alcyon 1 1
Northern Flicker
Colaptes auratus 1 1 2
Yellow-bellied Sapsucker
Sphyrapicus varius 1 1
American Tundra Peregrine Falcon
Falco peregrinus tundrius 1 1
Species Pre-2010 2010 2011 2012 2013 2014 2015 2016 2017 Total
number
of records

316
Eastern Wood Pewee
Contopus virens 2 2
White-eyed Vireo
Vireo griseus 3 3
Yellow-throated Vireo
Vireo avifrons 2 1 3
Philadelphia Vireo
Vireo philadelphicus 2 2 2 2 1 2 2 3 16
Red-eyed Vireo
Vireo olivaceus 12 7 4 (5) 7 (9) 7 (8) 8 (11) 6 (13+) 6 (7+) 11 (24+) 68
American Northern Shrike
Lanius borealis borealis 1 1
Ruby-crowned Kinglet
Regulus calendula 1 1
Tree Swallow
Tachycineta bicolor 3 1 1 5
Purple Martin
Progne subis 1 1 2
American Barn Swallow
Hirundo rustica erythrogaster 1 1 1 1 4
American Cliff Swallow
Petrochelidon pyrrhonota 1 1 2 (4) 2 2 1 1 (2) 10
Cedar Waxwing
Bombycilla cedrorum 1 1 1 3
Wood Thrush
Hylocichla mustelina 1 1
Hermit Thrush
Catharus guttatus 1 1 2
Swainson’s Thrush
Catharus ustulatus 1 1 1 1 4
Grey-cheeked Thrush
Catharus minimus 2 1 2 2 1 5 (7) 2 (3) 15
Veery
Catharus fuscescens 1 1
American Robin
Turdus migratorius 1 1 1 1 4
American Buff-bellied Pipit
Anthus rubescens rubescens 3 2 (3) 1 2 (3) 1 (2) 3 (7) 2 (5) 1 15
Summer Tanager
Piranga rubra 1 1 1 3
Scarlet Tanager
Piranga olivacea 3 2 1 2 (3) 2 (5) 1 4 15
Dickcissel
Spiza americana 1 2 1 4
Rose-breasted Grosbeak
Pheucticus ludovicianus 6 2 (3) 3 2 2 (7) 1 4 20
Indigo Bunting
Passerina cyanea 14 4 1 8 (12) 3 (4) 2 4 (10+) 3 39
Savannah Sparrow
Passerculus sandwichensis 1 1
Lincoln’s Sparrow
Melospiza lincolnii 1 1 1 3
White-crowned Sparrow
Zonotrichia leucophrys 1 1 1 3
White-throated Sparrow
Zonotrichia albicollis 1 1
Bobolink
Dolichonyx oryzivorus 5 2 3 3 1 1 1 1 17
Baltimore Oriole
Icterus galbula 3 2 (3) 2 7
Ovenbird
Seiurus aurocapilla 3 1 1 5 10
Northern Waterthrush
Parkesia noveboracensis 1 2 1 5 (6) 9
Species Pre-2010 2010 2011 2012 2013 2014 2015 2016 2017 Total
number
of records
Nearctic vagrants on Corvo, Azores, in 2005-17

317
Golden-winged Warbler
Vermivora chrysoptera 1 1
Blue-winged Warbler
Vermivora cyanoptera 1 1 2
Golden-winged x Blue-winged Warbler
Vermivora chrysoptera x cyanoptera 1 1
Black-and-white Warbler
Mniotilta varia 1 1 2 3 3 (4) 2 5 17
Tennessee Warbler
Oreothlypis peregrina 1 1 1 1 4
Common Yellowthroat
Geothlypis trichas 6 1 2 2 3 1 2 1 (3) 4 22
Hooded Warbler
Setophaga citrina 2 2 4
American Redstart
Setophaga ruticilla 3 1 1 2 7
Northern Parula
Setophaga americana 2 1 4 1 3 3 2 16
Magnolia Warbler
Setophaga magnolia 1 2 3
Bay-breasted Warbler
Setophaga castanea 1 1
Blackburnian Warbler
Setophaga fusca 1 1
American Yellow Warbler
Setophaga aestiva 1 1 2 1 5
Chestnut-sided Warbler
Setophaga pensylvanica 1 1
Blackpoll Warbler
Setophaga striata 1 1 3 1 1 (2) 6 (8) 13
Black-throated Blue Warbler
Setophaga caerulescens 3 1 2 1 7
Myrtle Warbler
Setophaga coronata 6 1 3 10
Yellow-throated Warbler
Setophaga dominica 1 1
Prairie Warbler
Setophaga discolor 1 1
Black-throated Green Warbler
Setophaga virens 3 1 1 3 8
Canada Warbler
Cardellina canadensis 1 1 1 3
Total records** 221 67 59 82 60 46 59 33 115
Total species 71 (*27) 38 37 42 36 24 35 26 48
Total landbird species 38 (*15) 20 22 26 24 16 27 14 29
Total waterbird species 33 (*12) 18 15 16 12 8 8 12 19
*Annual average 2005-09 (Alfrey et al 2010)
**Any deviation in the total number of records between this table and other references are due to the degree of ambiguity inherent in
assigning number of records compared with numbers of individuals
Species Pre-2010 2010 2011 2012 2013 2014 2015 2016 2017 Total
number
of records
Nearctic vagrants on Corvo, Azores, in 2005-17