What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
Cory’s Shearwater is Calonectris borealis; Scopoli’s Shearwater is Calonectris diomedea. Cory’s mainly breeds on islands in the Northeast Atlantic, while Scopoli’s mainly breeds in the Mediterranean. At sea, the most useful distinction is the white “tongue” on the underside of the outer primary, especially p10. A p10 tongue longer than 20% of the visible primary favors Scopoli’s; no white tongue on visible p8–p10 favors Cory’s. Birds in the 5%–20% interval should normally be recorded as Cory’s/Scopoli’s unless several supporting features point one way.
What the two names mean
These are two currently recognized species in the Calonectris shearwater group. Cory’s is Calonectris borealis, and Scopoli’s is Calonectris diomedea. Older books and websites may use “Cory’s Shearwater” for a broader, formerly polytypic species that included birds now assigned to Scopoli’s. An American Ornithological Society taxonomic proposal documented the split and transferred the name C. diomedea to Scopoli’s, leaving C. borealis for Cory’s. The British Trust for Ornithology (BTO) also currently lists Cory’s as C. borealis.
As an Amazon Associate I earn from qualifying purchases.
That history matters when you compare records. A sighting identified as “Cory’s Shearwater” in an older source may represent the former species complex rather than the species that modern checklists call Cory’s. Always check the scientific name and the date of the identification before comparing historical range or abundance statements. A historical “Cory’s” entry cannot automatically be converted to C. borealis without knowing which taxonomic treatment the observer used.
The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Quick comparison
| Feature | Cory’s Shearwater | Scopoli’s Shearwater |
|---|---|---|
| Current scientific name | Calonectris borealis | Calonectris diomedea |
| Main breeding region | Islands in the Northeast Atlantic | Mediterranean islands and coasts |
| Outer-primary underwing (p10) | No visible white tongue supports Cory’s; a short tongue can overlap | A white tongue longer than 20% of the visible p10 primary supports Scopoli’s |
| Intermediate p10 tongue | 5%–20% is an overlap zone | 5%–20% is an overlap zone |
| Outer greater primary coverts | Dark marks often occur on both gpc9 and gpc10 | More often one mark at gpc10 or no mark |
| Lesser secondary coverts | On average, denser dark markings | On average, less densely marked |
| Build and size | Extreme large or robust birds can support an identification | Extreme small or slimmer birds can support an identification |
| Geographic tendency | Atlantic breeding colonies; regular Atlantic migrant | Mediterranean breeding colonies; migrates into the Atlantic after breeding |
| Confidence from one feature | Not guaranteed; variation occurs | Not guaranteed; variation occurs |
The table summarizes tendencies, not a set of infallible tests. Flood and Gutiérrez’s 2021 field-separation study found substantial overlap, particularly in intermediate underwing patterns. The purpose of combining characters is to improve confidence while preserving an honest unresolved category when the view is inadequate.
#1 Best Overall
The most useful field mark: the p10 white tongue
Look at the underside of the outer wing when the bird banks. The primaries are the long flight feathers at the wing tip; observers commonly refer to the outer feathers as p8, p9 and p10, with p10 the outermost visible primary in the study’s measurements. A white “tongue” is a pale extension running inward from the white underside toward the darker outer portion of the feather. The practical comparison is the tongue’s length relative to the part of that primary you can actually see.
Think in proportions rather than millimetres. If only half of p10 is exposed because the wing is folded or another feather overlaps it, score the white extension against that visible half, not against an imagined full feather. Record which primaries were exposed. This prevents a hidden section from being mistaken for an absence of white.
Pattern that supports Cory’s
In Flood and Gutiérrez’s sample, birds with no white tongues on the visible portions of p8–p10 supported Cory’s Shearwater. This is a supporting pattern, not a guarantee: poor viewing angle, shadow, distance and partially hidden feathers can make a short tongue disappear. A brief flash of a white underwing is therefore not enough; wait for a banking sequence or use several photographs if available.
Recommended Free Tools
Pattern that supports Scopoli’s
A p10 white tongue longer than 20% of the visible primary supported Scopoli’s in the study. Estimate the proportion along the visible p10 shaft rather than judging the absolute length of a white patch. The comparison is most useful when the wing is fully spread or the bird banks enough for the outer underside to be seen clearly. A long, cleanly visible extension is more informative than a pale edge seen for a fraction of a second.
The 5%–20% overlap zone
A tongue occupying 5%–20% of visible p10 is not a reliable single-species character. Flood and Gutiérrez found that up to 2.5% of Cory’s and about 31.8% of Scopoli’s in their study sample fell into the shared 5%–20% categories. Those percentages describe the study’s measured birds and categories; they are not probabilities for every bird you see. In this interval, record “Cory’s/Scopoli’s” unless the other underwing marks, overall structure and geographic context combine to make one identification substantially more convincing.
Do not turn the 20% line into a false sense of precision. A photograph may not show the feather edge sharply enough to distinguish 18% from 22%, and perspective changes the apparent length. The threshold is useful when the view is clear, not when it requires guesswork.
How to use the rest of the underwing
Lesser secondary coverts
Cory’s tends, on average, to have denser dark markings on the underwing lesser secondary coverts. This is a comparative impression across the coverts, not a countable threshold. Lighting can alter how much mottling you see, and individual birds vary, so use the pattern to support a p10 assessment rather than replace it. Note whether the coverts look broadly and repeatedly marked, rather than focusing on one isolated fleck.
Outer greater primary coverts
Inspect the outer greater primary coverts, the row of coverts bordering the bases of the outer primaries. Dark marks on both gpc9 and gpc10 occur more often in Cory’s. Scopoli’s more often shows a single mark at gpc10 or no mark. Because these are frequency differences, an individual Scopoli’s can show the Cory’s-leaning pattern and vice versa. View the row when the wing is open; a closed wing can hide one or both relevant coverts.
Rank #2
- The Sibley Guide to Birds has quickly become the new standard of excellence in bird identification guides, covering more than 810 North American birds in amazing detail. Now comes a new portable guide from David Sibley that every birder will want to carry
Size and build
Very large, heavy-looking birds can support Cory’s, while an unusually small or slender bird can support Scopoli’s. The size and structural distributions overlap substantially, and judging build from a moving bird at sea is difficult. Avoid statements such as “large equals Cory’s.” Use structure only when the difference is extreme and the wing pattern agrees. Perspective, wind posture and the angle of the wings can all make the same bird appear broader or narrower.
A step-by-step identification workflow
- Confirm that the bird is a large Calonectris shearwater. Wait for a view of the long outer wing and underwing before attempting the species-level split. Do not force an identification from a distant silhouette.
- Choose the best banking view. Follow the bird until the underside of the outer wing is visible. Note whether p8, p9 and p10 are exposed; a feather hidden by overlap cannot be scored as if it were visible.
- Check p10 first. Look for a white tongue and estimate its length as a percentage of the visible p10 primary. No visible tongue supports Cory’s; more than 20% supports Scopoli’s; 5%–20% is intermediate.
- Check the neighboring primaries. A consistent absence of white tongues across visible p8–p10 strengthens a Cory’s assessment. A clearly extended p10 tongue is more informative than a vague pale edge elsewhere.
- Assess the coverts. Record whether lesser secondary coverts look densely marked and whether gpc9 and gpc10 both carry dark marks. Treat these as corroboration.
- Consider structure only at the extremes. A distinctly robust or distinctly slight bird may help resolve a borderline pattern, but ordinary variation should not be used to override the wing evidence.
- Add geographic and seasonal context. Breeding region, migration timing and location can change the prior likelihood, but they cannot settle every sighting because the species’ ranges overlap during migration.
- Assign a confidence level. If the p10 measurement is intermediate or the wing was poorly seen, submit a combined Cory’s/Scopoli’s record rather than an unsupported species call.
For a photograph, apply the same sequence at full resolution. First locate p10, then mark the visible feather length, and only afterward inspect coverts and structure. Cropping tightly around the outer wing can help reviewers, but retain the uncropped image so the bird’s posture and scale remain available.
A practical confidence scale
A simple three-level description keeps records consistent. “High confidence” is appropriate when p10 is clearly visible, the tongue is absent or exceeds 20%, and supporting marks do not conflict. “Moderate confidence” fits a mostly clear view with one supporting feature but some uncertainty in proportion or feather exposure. “Combined or unresolved” is the honest choice when p10 lies in the 5%–20% interval, the wing is partly hidden, or the evidence points in opposite directions. This is not a failure to identify the bird; it accurately describes what the observation can support.
Why geography helps but cannot decide every record
Scopoli’s breeds mainly in the Mediterranean, whereas Cory’s breeds mainly on islands in the Northeast Atlantic. That broad pattern is useful before you examine a bird, especially near colonies or during the breeding season. It is not a hard boundary. Scopoli’s migrate into the Atlantic, where they can occur among Cory’s, and the field-separation study notes rare Cory’s breeding in the Mediterranean as well as rare hybridisation.
CIESM reports that Scopoli’s adults leave Mediterranean breeding grounds in October and move south through the Atlantic. Some remain in the Mediterranean during winter. These movements explain why an Atlantic observation does not automatically identify Cory’s and why a Mediterranean observation does not automatically identify Scopoli’s. A bird encountered far from a colony may be a migrant from the other breeding region.
For Britain, the BTO describes Cory’s as a scarce passage migrant, with most records in July and August. That seasonal pattern is useful for planning observation, but a date alone is not a species diagnosis. Weather-driven movements can also concentrate seabirds in places where they are not normally expected.
Reading range maps without overinterpreting them
Cornell’s combined Cory’s/Scopoli’s range map uses a breeding-season window of 10 May–26 July and a post-breeding migration window of 2 August–27 December. Those dates describe the combined data product; they do not cleanly separate the two species. A map that merges the taxa can show where the complex occurs while concealing the species-level overlap that makes individual identification difficult. Treat map shading as a guide to when and where to look, not as a substitute for the underwing.
Quick wins for a faster PC:
Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →The Cape Verde Shearwater is a separate species, endemic as a breeder to Cape Verde. It should not be treated as a third name for the Cory’s–Scopoli’s pair or folded into their split taxonomy.
Rank #3
- Dimension(in): (L) x (W) x (D)
What the 2021 field study actually tested
Flood and Gutiérrez analyzed 752 Cory’s Shearwaters and 814 Scopoli’s Shearwaters. Their work is valuable because it tests combinations of visible field characters rather than presenting a single plumage mark as diagnostic. The p10 white-tongue measurement emerged as the strongest practical distinction in the cited material, while covert markings and extreme structure supplied additional support.
The sample sizes also explain why the percentages should be handled carefully. A study category can reveal overlap in a measured sample without providing a universal error rate for all seas, seasons, ages or viewing conditions. Weather, distance, wing position and the proportion of the feather that is visible all affect what an observer can score. The study’s conclusion that most birds can be separated at sea does not mean every bird can be assigned confidently.
The study’s approach also argues for recording the actual observations, not merely the final label. A note such as “p10 tongue approximately 12% of visible feather; gpc9 and gpc10 both marked; lesser coverts dense” can be reassessed. A note that says only “looked like Scopoli’s” cannot.
How to record a borderline sighting
A good record preserves the evidence instead of hiding uncertainty. Write down the visible p10 proportion, which of p8–p10 were exposed, the gpc9/gpc10 marks, the density of lesser-covert markings and any structural impression. Add location, date, direction of travel and whether the bird was in a breeding colony area or on migration. If the tongue is in the overlap interval or the view was brief, use the combined label “Cory’s/Scopoli’s.”
This approach is more informative than choosing a species because one field mark seemed close to a threshold. It also makes later review possible if photographs, video or additional observers provide a better view. A combined record is an accurate result when the available evidence does not resolve the species. If several birds are present, document each bird separately when their wing patterns differ rather than assuming the flock belongs to one species.
Common identification mistakes
- Using the common name without checking taxonomy. Older “Cory’s” accounts may refer to the former broad species concept. Confirm whether the source means C. borealis or the historical complex.
- Treating a pale flash as a measured p10 tongue. Score the feather only when its visible length can be compared with the visible primary.
- Calling every intermediate bird Scopoli’s. The 5%–20% interval is explicitly shared; use a combined identification when supporting evidence is weak.
- Letting geography override plumage. Migrating Scopoli’s occur in the Atlantic, and rare Cory’s breed in the Mediterranean.
- Relying on body size alone. The distributions overlap and field judgements of build are especially vulnerable to perspective and distance.
- Reading a combined map as two separate maps. Cornell’s product combines the species and should not be used as a definitive species-level boundary.
- Scoring a hidden feather as blank. Absence of a visible tongue means no tongue on the exposed feather; it does not prove that a concealed section lacks white.
- Ignoring the direction of light. Glare, shadow and sea spray can erase fine mottling or create a false pale edge, so seek more than one view.
Population context
CIESM gives an estimate of 141,000–223,000 pairs for Scopoli’s, but the retrieved guide page does not show a publication year for that figure. Treat it as an undated estimate rather than a current-year population total. No equivalent current estimate should be inferred for Cory’s from that number. Population figures and taxonomic boundaries can change as colonies are surveyed and species treatments are revised, so quote the estimate with its source and date status whenever it is used.
Choosing equipment and references
Because the key character is on the moving underside of the outer wing, binoculars with a stable view and a camera or phone capable of freezing a banking bird can improve your records. Image stabilization, a fast shutter speed and a burst of several frames are more useful than extreme digital zoom. Keep the original photographs with your notes; compression and heavy cropping can remove the feather edges needed to judge a 5%–20% difference.
Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsA field guide that illustrates underwing patterns and explains the split is useful as a reference category, but no single plate should replace the p10 measurement and the overlap rule. Compare multiple views when possible, and ask another observer to assess the same image independently. Agreement is strongest when both observers can see the same exposed primaries and describe the proportion rather than simply choosing a species name.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently asked questions
Are Cory’s and Scopoli’s Shearwaters the same species?
They are treated as two species today: Cory’s is Calonectris borealis and Scopoli’s is Calonectris diomedea. Older references may combine them under Cory’s Shearwater because the split was not yet applied.
Can I identify one by range alone?
No. Cory’s mainly breeds in the Northeast Atlantic and Scopoli’s mainly in the Mediterranean, but Scopoli’s migrate through the Atlantic, rare Cory’s breed in the Mediterranean, and rare hybridisation is reported. Range is context, not a guarantee.
What exactly is the p10 tongue?
It is the white extension running inward on the underside of the outermost visible primary. Judge its length as a percentage of the visible p10 feather, not as an absolute patch size.
What does a tongue under 5% mean?
A very short or absent tongue generally supports Cory’s, provided p10 is genuinely visible. If the feather is hidden or the image is too poor to measure, do not treat the apparent absence as decisive.
What should I do with a 5%–20% tongue?
Use the combined identification Cory’s/Scopoli’s unless other characters strongly resolve it. The overlap is real, and forcing a species label can be less accurate than preserving the uncertainty.
Are dark covert marks diagnostic?
No. Cory’s averages denser dark markings on the lesser secondary coverts, and marks on both gpc9 and gpc10 are more frequent in Cory’s. These are supporting frequency differences, not guaranteed individual traits.
Does a larger bird have to be Cory’s?
No. Extreme robust build can support Cory’s and an unusually slim build can support Scopoli’s, but size distributions overlap and perspective makes structure difficult to judge at sea.
Free tools Windows power users keep installed
One-click scans. No signup required.
When are Cory’s most likely in Britain?
The BTO describes Cory’s as a scarce passage migrant, with most British records in July and August. That timing helps plan observation but cannot replace an underwing assessment.
Is Cape Verde Shearwater part of this split?
No. Cape Verde Shearwater is a separate species and breeds endemically in Cape Verde. It should not be used as another name for either Cory’s or Scopoli’s.
Bottom line for observers
Start with the scientific names and breeding regions: Cory’s is Calonectris borealis of the Northeast Atlantic, and Scopoli’s is C. diomedea of the Mediterranean. Then inspect the underwing, giving priority to the p10 white tongue. No visible tongue across exposed p8–p10 leans Cory’s; a tongue over 20% of visible p10 leans Scopoli’s. Between 5% and 20%, combine the underwing clues with extreme structure and geographic context, and leave the record as Cory’s/Scopoli’s when the evidence remains mixed.
Frequently Asked Questions
Are Cory’s and Scopoli’s Shearwaters the same species?
They are currently treated as two species: Cory’s is Calonectris borealis and Scopoli’s is Calonectris diomedea. Older sources may combine them under Cory’s Shearwater.
Crashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minutePC Slower Than It Used to Be?
A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Can geography alone identify the species?
No. Breeding regions differ, but migration brings Scopoli’s into the Atlantic, rare Cory’s breed in the Mediterranean, and hybridisation is reported. Use geography as context alongside underwing marks.
What does an intermediate p10 tongue mean?
A white tongue covering 5%–20% of visible p10 falls in the overlap zone. Record Cory’s/Scopoli’s unless several supporting features clearly favor one species.
Are covert markings diagnostic?
No. Cory’s averages denser lesser-covert markings and more often has marks on both gpc9 and gpc10, but individual birds vary. Use these traits only as corroboration.
When are Cory’s seen in Britain?
The BTO describes Cory’s as a scarce passage migrant, most often recorded in July and August. Timing is useful context, not a substitute for field marks.
The Bottom Line
Use p10 first, context second, and uncertainty honestly. No visible white tongue on exposed p8–p10 supports Cory’s; a p10 tongue longer than 20% supports Scopoli’s. The 5%–20% interval is shared, so a combined Cory’s/Scopoli’s record is often the most accurate result.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
Leave a Reply