Entomology, Ornithology & Herpetology: Current Research
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The Caspian Tern Hydropogne caspia in Tanzania

Neil Edward Baker^{* *}Correspondence: Mr. Neil Edward Baker, Tanzania Bird Atlas, Tanzania, Tel: +255 753-513603, Email:

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Caspian Tern

Caspian Terns usually feed singularly close to shore or over the reef using salt pans for roosting at high tide when they concentrate on the bunds and are more easily counted. Occasionally sea water is pumped into the salt pans at high tide and the terns exploit the small fish that become available. This allows photographic opportunities such as this hovering individual. Note the dark-centered secondaries and dark tail feathers that suggest a first-winter bird, as do the much worn outer primaries. Note the active wing moult with 6 old outer primaries on the left wing contrasting with the paler grey new inner primaries. Egg laying in the Malagasy Region occurs from late May through to August when counts on the Tanzanian coast are at their lowest (Figure 1).

Figure 1. Bird hovering (Caspian Tern 16 March 2014 Bagamoyo, Tanzania).

Since at least there has been an increasing non-breeding population of Caspian Terns along the coast of Tanzania which likely now numbers more than 600 birds. But, where might these birds have come from? There are only three possibilities [1].

• Birds from Western Palaearctic breeding grounds that are now wintering further south.

• Birds from an expanding South African breeding population that are now wintering further north or

• Birds from the poorly known breeding population on Madagascar.

Baker [2] speculated that “They are unlikely to be from the southern populations in Madagascar and South Africa but this cannot be ruled out”. This species has, until quite recently, been a rare bird on the coast of Tanzania. Britton and Brown [3] could only refer to Harvey [4] who saw a single bird in Dar es Salaam on 17 December 1972 and two birds at Bagamoyo on 5 March 1973.

Britton [5] had nothing to add to these two records and only reported two records for coastal Kenya south of Mida Creek and gives a peak wintering population from Malindi at 25-35 birds in December-February. Fuggles-Couchman [6] adds two records from near Dar es Salaam in October 1956. The Tanzania Atlas Database currently holds 187 records for this species from 45 observers. Table 1 shows the details of 157 of these records. The other records being duplicates or no count so of lesser value.

Table 1: Observations for Tanzania from the Tanzania Bird Atlas Project database.

Breeding season records for Madagascar include Nosy Mborono (SW) June-August, Aldabra April-August, Nosy Tsara (N) from March, Nosy Foty (NW) May or June. Madagascar population estimates are given as 1,000 – 2,000 and 960 – 1,900 and the highest non-breeding count is of 200 birds [7]. Table 2 gives the number of records in each month with month maximums.

Table 2: Number of observations and highest counts for each month for Tanzania from the TBAP database.

The breeding season fits well when birds are scarce in Tanzania. Stafford and Hawkins [7] give age at first breeding at 3 years or more which could account for low numbers off the Tanzania coast during the Malagasy breeding season.

Table 3 gives extra records taken from ebird. While this data adds to the general impression of a widespread species only the targeted survey of the Rufiji Delta in January 2017 includes significant numbers.

Table 3: ebird records for Tanzania.

Note the lack of even casual records for August and September. Table 4 gives ebird records for Kenya. This maximum count of 30 birds suggests no changes in this wintering population since the 1970s. These Kenyan records do not suggest the 600 or so birds wintering on the Tanzania coast originate from the Western Palaearctic. Table 5 gives ebird records for Mozambique.

Table 4: ebird records for Kenya.

Table 5: ebird records for Mozambique.

These few records do not suggest that the 600 birds wintering in Tanzania originate from the South Africa breeding population. We do not know the origins of these birds. It was earlier assumed they are from the Western Palaearctic (rather than South Africa) but the nearest large breeding populations are in the Black Sea with a 1% threshold of only 40 birds and the Caspian Sea with a 1% threshold of 120 birds [8]. There has been a small increase in sightings in Somalia with only 4 records prior to 1978 and 47 between then and the late 1990s [9]. Birds are present in low numbers throughout the year in Ethiopia and Eritrea [10] and there is a small breeding population on the coast of Eritrea with 103 pairs present on 45 islands in the northern winter [11]. Nikolaus [12] mentions “up to 300 on autumn passage” for the Sudan and that Swedish and Russian ringed birds have been recovered at Jebel Aulia and Sennar dams.

However, the lack of records from the well watched Kenya coast (Table 4) where no large flocks are known suggest that those now spending their non-breeding season on the Tanzania coast are not from the Western Palaearctic. Note that the current maximum flock size for the Kenya coast is, at 30 birds, the same as it was in the 1970s.

Ebird (accessed 30 September 2018) hosts 13 records for Tanzania that are not (yet) in the Atlas database (Table 3). During a waterbird survey of the Rufiji Delta in January 2017 counts of 76 and 46 have been submitted to ebird but not yet published (SN). Only one other count is of more than 10 birds, 14 off Mkwaja on 24 November 2017 (ST).

Clearly the coast of Tanzania is now hosting close to 200 birds in the Rufiji Delta, 160 in the vicinity of Bagamoyo and at least 100 in the vicinity of Saadani NP and 100 close to Pangani (Pembe Abwe). Given that there are smaller numbers at many other sites a non-breeding population close to 700 birds is likely [2]. The Rufiji Delta (with Mafia Island) is already a designated Ramsar site. The data presented here suggests that the salt pans in the valley of the lower Ruvu River flood plain qualify as a shadow Ramsar site for this species and so may those in Saadani NP when more effort has been made to count them on a regular basis. It should be remembered that high counts are only likely at roost sites during periods of high tide which helps explain lower counts during the months of peak abundance.

Safford and Hawkins [7] consider it a resident breeder on Madagascar, the Aldabra archipelago and on Europa. Ebird data for Madagascar lists 41 localities with only 3 serious counts of 10, 16 and 25 birds.

Peak counts for the Tanzania coast (Table 2) are from November through March. This suggests that many Malagasy birds could now be spending their non-breeding season on the coast of Tanzania. If this is correct then any count of 15 birds can be considered as meeting the Ramsar 1% criteria with at least 2 sites holding more than 10% of the population. Figure 2 details geo-referenced observation and the 4 main roost sites. A question for ornithologists and conservationists in Madagascar is WHY have these birds changed their non-breeding sites? The African Eurasian Waterbird Agreement (AEWA) does not consider any African populations as migratory. This may now have to change and I would suggest a tighter population estimate is required for the Malagasy population. It may well be that more than 50% are now wintering on the coast of Tanzania.

Figure 2. Geo-referenced localities for Caspian Tern on the coast of Tanzania. The 4 known high tide concentrations are detailed. There are likely to be others that have not yet been located.

Acknowledgements

The following are warmly thanked for contributing to the Atlas. There are names missing here who contributed as team members during the 2005 Waterbird count, for these omissions I apologize. Your counts are included under the name of the team leader.

Steven Adinall, Zul Bhatia, Nick Borrow, Martin Butterfield, Steven Cavers, Andreas Christensen, John Dixon, Melissa Eager, N.R. Fuggles-Couchman, A. Garpebring, Thomas Gottschalk, Olivier Hamerlynck, Jude Jarvis, Thomas Johansen, Jasson John,Thomas Lehmburg, Pete Leonard, Riaan Marais, Hans Meltoft, Kelvin Moon, Athanas Mwakyelu, Oliver Nasiwa,Timme Nyegaard, Paul Oliver, Jan Olssen, Edward Pollard, Fiona Reid, Allan Sander, Itai Shani, Kath Shurcliff, Mary Simmonds, Jez Simms, Chris and Tilde Stuart, Walter Suter, Ole Thorup, Simon Totterman, Martii Vatulainen, Friedemann Vettel, John Whittle, Barry Whitmore, Hagai Zvulun.

Szaboicz Nagy and Simon Thompson are thanked for submitting their counts to e-bird.

References

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