This is an article that I wrote for the Oxon Birding website, as part of the March 2018 bird news update:
Black-necked Grebe is a scarce passage and winter visitor to Oxfordshire but has been recorded in every month of the year and in every year during this period, except for 2008, when there were no records in the county. Scanning a reservoir or gravel pit in spring and finding a summer-plumaged Black-necked Grebe may be the highlight of spring migration for birders in a land-locked county such as Oxfordshire. This article looks at the pattern of occurrence for Black-necked Grebe in the 14 years from 2000 to 2013 inclusive.
As many observers can submit records for each individual bird present, using the total number of records per year on the OOS database does not provide an accurate reflection of the actual number of birds present. The table below was calculated by examining the OOS reports from 2000-2013 and determining the likely number of birds present in each month, rather than the total of submitted observer records. A conservative approach to numbers was taken if birds were reported from nearby locations on consecutive dates, so the real numbers of birds involved may well be higher than this estimate. Figure 1 shows, for example, that there has been a total of 5 Black-necked Grebes recorded in the month January in the years 2000-2013:
Spring records:
There is a clear spike in records beginning in March and increasing in April. These records are of wintering birds returning to their breeding grounds in northern and central England. Black-necked Grebes were recorded in the month of March in three years and in the month of April in six years during the fourteen-year period between 2000 and 2013. There were cumulative totals of 9 birds recorded in March and 18 in April (see figure 1). Birds recorded in early March, such as the birds pictured below at Dix Pit on 7th March 2018, are in predominantly winter plumage. By the end of April most Black-necked Grebes will be in full breeding plumage.
However, the totals for March and April are inflated by two significant flocks of Black-necked Grebes: 5 birds together at Dix Pit on 6th March 2002 and 7 summer plumaged birds together at Farmoor Reservoir on 27th April 2012. Some of these birds remained for a number of days and gave fabulous views, even performing courtship displays on occasions:
Breeding season records:
There is only one May record (of two birds on 24th May 2013 at Farmoor) and a single record from June: a bird in full breeding plumage seen at Standlake on 4th June 2000. This is the only June record of Black-necked Grebe in Oxfordshire (OOS Annual Report, 2000) and at time of year when breeding birds are usually on territory, hence the paucity of Oxfordshire records in early summer. This bird could have been a very early failed breeder, but perhaps is more likely to have been a breeding bird dispersing in search of potential nesting habitat. This species is noted for “widespread aerial reconnaissance” (CBWP).
Post-breeding records:
The gradual increase in records from July through to September correlates with the arrival of failed breeders and post-breeding dispersal. Failed breeders may leave breeding ponds as early as July, whilst post-breeding dispersal typically occurs from mid-August (Migration Atlas, BTO). This accounts for the gradual rise in Oxfordshire records in the autumn period. In eight of the fourteen years analysed Black-necked Grebes were recorded in September, making this the most productive month overall for records in Oxfordshire, even though the overall number of birds found in September, sixteen, is eclipsed by the total found in April, eighteen.
Winter records:
In the western palearctic Black-necked Grebes winter within the breeding range, with movements to the coast when inland water-bodies freeze. CBWP states that most birds are on their wintering grounds by November and that the UK wintering population may receive birds from continental Europe that shelter in the estuaries and inland water bodies of south-east England. Winter records in Oxfordshire increase from October to December (see figure 1) and could conceivably be from both Black-necked Grebes that breed in the UK or from birds that have migrated to southern England from breeding grounds in continental Europe.
A global footnote:
The movements described above are trivial compared to the migration of Black-necked Grebe in other parts of its global range. Hundreds of thousands of Black-necked Grebes are thought to winter in the south Caspian Sea off the coast of Iran, with tens of thousands on Turkish lakes in winter (BTO, Migration Atlas).
In North America nearly 1.5 million Eared Grebes (the name by which Black-necked Grebes are known in the west) migrate to Lake Mono in California where the adults undergo a complete moult and the juvenile birds a partial moult. The birds at Lake Mono more than double their weight on their autumn staging grounds, their chest muscles shrink as their bodies favour rapid fat deposition. Eared Grebes are rendered flightless during this period and are flightless for up to 9 or 10 months of the year, the longest flightless periods for any species bird capable of flight in the world (Cornell 2017). Eventually the brine shrimp population, on which the vast numbers of grebes feed, collapses. The grebes, by now having completed their moult, migrate to the coast at night. The nocturnal movements of so many birds can leave them prone to severe weather-related incidents, such as this report of some 4,000 Eared Grebes that landed in a Walmart car park in a storm in Utah in December 2011. In Europe both diurnal and nocturnal migration of Black-necked Grebe has been recorded, in contrast to the purely nocturnal migration of Eared Grebe in North America (Migration Atlas, BTO).
References:
“Bird Atlas 2007-2011”, BTO, 2013.
“The Birds of the Western Palearctic”, concise edition (CWBP); Snow & Perrins; OUP 1998.
“The Migration Atlas”, BTO, 2002.
The Cornell Lab of Ornithology; “Eared Grebe” website; 2017; [https://www.allaboutbirds.org/guide/Eared_Grebe/overview]
Weblinks to news items on the mass downing of thousands of Eared Grebes in Utah in 2011:
https://www.ksl.com/?sid=18498424
https://www.cbsnews.com/news/more-than-4000-birds-crash-land-in-parking-lot/