Sand Martin colony during the pandemic: Andrew Moon

Andrew Moon (all photographs – Andrew Moon)

Preamble

Sand Martins really are one of the true harbingers of spring and often arrive back in this country when the weather is decidedly inclement, usually from late March onwards, but sometimes even earlier. the first birds to arrive back in the uk are frequently found feeding low over lakes or reservoirs, quite often in cold, wet and windy conditions. they are clearly very hardy birds, but occasionally there can be quite high mortality if they are suddenly faced with a severe cold snap on migration, or when they arrive back in Britain.

Distribution and Migration

the worldwide range of the Sand Martin is truly colossal and extends right across eurasia, Asia and North America (where it is known as Bank Swallow), with the winter range reaching as far south as southern Asia, Africa and South America. their generic name riparia is derived from the Latin word riparius meaning ‘of or belonging to a bank of a river’. the only comparable swallow or martin with a larger breeding and winter range is the (Barn) Swallow (Hirundo rustica), with the latter also wintering in Australia. According to Bto statistics there were between 71 and 225 thousand Sand Martin nests in the uk in 2016, with the majority nesting north of a line from the Severn to the Wash. this is just 4% of the european population, which is put at between 2.4 and 4.5 million pairs. Both of these totals are, however, well below population sizes from 20-30 years ago; indeed, estimates of the uk breeding population exceeded one million pairs in the 1960s. Despite that rather gloomy statistic it would seem as though numbers in the uk have actually increased slightly since 1994.

British birds, following extensive ringing studies, are known to migrate south in autumn through france and Spain and then on into Morocco, continuing south and then east to their main wintering range in the semi-arid Sahel zone of western Africa (approximately from Senegal in the west to Burkina faso and Ghana in the east). the Sahel lies to the south of the Sahara Desert, but to the north of the humid, tropical savannahs, and extends from the Atlantic in the west to the red Sea in the east. this area is clearly one prone to a fluctuating climate and, indeed, severe droughts in the Sahel region in the winters of 1968-69 and 1983-84 led to huge declines in the uk population. fortunately, they are prolific breeders and have been able to make up many of the losses in subsequent years. in the spring birds return north to Britain on a more easterly route, firstly across the Sahara, then northwards via tunisia, italy and france.

Studies have also revealed that they are diurnal migrants, so the need to lay down large fat deposits (unlike most nocturnal migrants) is not so important, although this may not be the case if they have to cross the Sahara Desert. interestingly, following the Sahel drought there is evidence that at that time their migration route shifted further east, instead of following the west coast of northern Africa, as had been the case originally.

it is known, again from ringing studies, that small groups of birds (at least) migrate together, frequently from the same colony, so it is possible that some birds stay together throughout the year. As only a tiny proportion of birds are ringed it would be difficult to extrapolate this further, but Sand Martins are most often seen in flocks so perhaps colony-faithfulness may extend to a wider attachment throughout the year. this is not to suggest that they did not mix with other Sand Martins from across europe, either in their winter quarters or on migration. As mentioned above, Sand Martins are frequently faithful to their natal colony, although that fidelity is slightly stronger in adults than juveniles. Nevertheless, those birds that do move to another colony seldom do so if it is more than ten kilometres away, with no more than 6.5% of adults moving to another colony between ten and 99 kilometres away in subsequent seasons, with slightly more juveniles (11%) doing so. A lot of the statistics listed above arose from a huge ringing programme

undertaken by the Bto in the mid-1960s which reached a peak of 75000 birds ringed in 1966.

As might be expected mortality is highest amongst juvenile birds in the autumn as they migrate south, but conversely mortality is highest amongst adults in spring, partly because they can arrive back at their breeding area in poor weather. Survival rates vary but are suggested to be close to 30-50% for adults and 20-40% for juveniles; this means that on average, they only live for a few years. the oldest adult in the uk survived at least 7 years and 9 months.

Breeding status in Suffolk

it is quite difficult to assess the actual numbers of breeding birds before 1991, the species was seldom mentioned in Suffolk Bird reports prior to that year and if there were any reports they usually related to early or late migrants. Nevertheless, there is no reason to think that it was not an abundant breeding bird in earlier times. Piotrowski (2003) states: ‘Up until 1968, there were large numbers of the species present in many areas’. A county-wide census in 1991 amassed a minimum total of 2646 breeding pairs, with the largest colony being of 1443 pairs in the cliff faces between covehithe and Southwold. these coastal sand cliffs still remain a very important breeding habitat in Suffolk, although they have always been in a constant state of flux with the ever-present risk of cliff falls providing a real hazard for birds, as well as humans. More recent annual counts of breeding birds have reached 1113 in 2002, 1053 in 2010 and, more encouragingly, 1063 in 2018, with the majority of these along the coast. No doubt the prevailing weather, especially in spring, can seriously impact on the number of birds that arrive in Suffolk to breed each year.

Breeding

Sand Martins nest primarily in vertical sandy cliffs, quarries or sand pits, although they will adapt to use artificial nest sites; indeed, i have watched nesting pairs using drainage holes in canal banks. Whilst many Sand Martins are very faithful to their natal colonies, they can also be very fickle and will abandon a colony one year and move elsewhere, before returning to the original colony the following year! usually, they will excavate a new nest hole each year, but many of the existing holes at Wenhaston were actually re-used (although further excavation may have taken place), whilst many new holes were created. these holes can be up to one metre in length, sloping upwards with a small nesting chamber at the end to which they add nesting material such as small twigs, roots and other small dry plant items which are then lined with feathers. Sand Martins, typically, lay four or five eggs (although they are known to lay up to seven) and incubation normally takes about 14 days, with fledging taking a further 19 days.

Past studies have revealed that breeding success is correlated with the length of the burrow, with those nesting in holes of less than 70 centimetres in length, being less successful (50%) than those that nest in holes longer than 70 centimetres (73%). it may be the case that more mature and experienced birds are quite likely to have longer nest holes and it is well known that older, more mature birds, return to the nesting colonies first and tend to choose burrows higher up on the face of the cliff, which are more likely to be safe from predators. the sand pit face at Wenhaston is mostly near-vertical, so the colony is largely safe from the obvious predators like Weasel (Mustela nivalis), Stoat (Mustela erminea) and Brown rat (Rattus norvegicus). Nevertheless, opportunistic red foxes (Vulpes vulpes), which have an earth nearby, would soon snaffle any youngsters that were unfortunate enough to fall out of their nests.

Wenhaston Black Heath

Prior to the winter of 2014/15 the sand pit at Wenhaston Black Heath was completely choked with gorse and the sand face was barely visible. in that winter the pit was cleared of extensive degenerate gorse and the sand face exposed. it seems very unlikely that Sand Martins would have bred here historically, or at least not for many years beforehand. All this sterling work was carried out by the Suffolk Wildlife trust (SWt) and the Wenhaston commons Group (WcG).

2016 and 2017

the first year that birds bred was in 2016 when about 25 pairs nested and up to 80 birds were seen around the pit, although breeding success was unknown. the following year, in 2017, the first birds arrived back on April 6th and once again they bred successfully; my notes for June 19th 2017 state that: ‘50 plus birds seen with many young in nest holes.’ it seems likely that a number of young were raised successfully as several nests were still in use by the end of July, although it is not known whether or not these were second broods.

2018 and 2019

in 2018 the first three birds arrived back at the quarry on April 18th, but there were very few sightings during the spring. the last five birds were seen on June 11th and i didn’t witness any attempts to enter the nest holes. unfortunately, 2019 proved to be another blank year with just seven birds being seen on April 20th (although they were exploring nest holes), one on 30th, two on May 22nd and finally 12 birds on June 27th, which were also seen to explore nest holes.

2020

following a severe gorse fire on the north and east side of the quarry in the autumn of 2019, much of the older stands of gorse bushes had been decimated. once again SWt and WcG stepped in and grubbed out much of the old root systems of these bushes, leaving those sides of the pit much more open. A low fence was also put in place as a safety measure to deter human visitors and provide some privacy for the birds.

Because we were all affected by the countrywide lockdown imposed on us on March 23rd with the instruction to ‘stay local’ it seemed a very logical step and an ideal opportunity to spend some time studying the local Sand Martins, as well as other birds and insects in the immediate area close to our house. the first Sand Martins arrived back on April 5th and immediately started exploring the disused holes in the sandy cliff in the quarry. unlike in 2019, these birds stayed around and were soon joined by others, increasing to ten by April 15th, by which time five holes were being explored, and 20 on 22nd, with ten holes in use. they soon started collecting nesting material (small roots and other plant material) from nearby slopes (where the old gorse stands had been). By the end of the month their numbers had increased to at least 40 birds and 15 nest holes were in active use. As we entered May it was becoming more difficult to count accurately the number of birds, but with 46 holes in use by May 15th it was safe to assume that 46 pairs were nesting (i.e. at least 92 birds). the first juveniles could be seen congregating at the front of five nest holes by June 15th, increasing incrementally to 19 nest holes by June 20th, 40 on June 30th and then 45 on July 2nd, involving at least 102 youngsters on the latter date (from at least 45 pairs). By July 11th most of these youngsters had left the nest holes and all these would have been first broods.

it seems incredible that a pair of Sand Martins, having raised a brood of youngsters, would contemplate going through the whole process again, but that is exactly what happened! By August 12th at least 19 holes were in use, of which 11 were in holes that hadn’t been used before. Whether any of those birds that nested in these 11 holes were nesting for the first time, or whether they were birds that had already raised a brood successfully (or failed) in another hole, is impossible to say. there is no question that many of the adults feeding young of their second broods looked, not surprisingly, rather tatty!

Another 42 young were raised from the second broods, making a grand total of at least 144 youngsters during 2020 – a fantastic achievement – particularly bearing in mind that this is a minimum total and doesn’t allow for nests not visible from the footpath, or those holes in which i did not see any young and which may have fledged unseen. the majority of broods held two or three youngsters, but three of the first broods had four young and one even had five. Whilst it was quite comical watching the young fighting to get to the edge of the nest hole to attract the attention of their parents, inevitably the weaker siblings probably lost out. of those that had two broods, the best hole was one that had three young in the first brood and then four in the second brood and another had three and three. to raise seven and six youngsters, respectively, from single holes in one year is quite astonishing and a tribute to the diligence of the parents.

However, there is a slight twist to this tale which is that second broods are not necessarily with the same partner or even in the same nest; furthermore some females leave their first broods before they are fully fledged, to start a second brood with another male, presumably leaving the original male to carry on feeding them! there was no way of knowing whether any of this happened at Wenhaston, but either way it still leaves one in awe of their dedication to raising young.

Alan Miller managed two sessions to trap and ring birds at the colony during the summer, so it would be interesting to perhaps learn of their onward travels or maybe even see them return to the colony. Alan had also managed to trap a few birds in 2017 and one of these birds had been ringed as a juvenile at Minsmere in 2014, a clear indication of a bird remaining faithful to the general area, if not exactly the same colony. Some of the birds that i managed to photograph collecting nesting material were carrying rings, although unfortunately i wasn’t able to read the ring numbers.

Method

i realised that the only way to keep track of the number of nest holes in use, as well as the number of young fledging, was to take a series of photographs of the face of the sand pit. this then allowed me to annotate the printed image with • the number of holes in use • the number of young present • the number of new holes being excavated i was then able to monitor how many new young were appearing at the nest holes, as well as how many new holes were being excavated by updating the annotated colour print-out on every visit (black and white prints were indecipherable!). As quite a few new holes were appearing i had to keep retaking photographs of the face of the pit so that i could keep up to date with the ongoing developments. Additionally, because the nesting holes were spread out across three separate faces of the pit face it meant that i couldn’t just take one picture, i had to take three or four! equally some nests were just not visible from the footpath and i would estimate that maybe 15% of the nest holes were missing from the analysis because, although i could see birds entering the face, i couldn’t actually see the holes. the likelihood is that the final number of pairs and young is higher by approximately 15%. it is fair to say that the monitoring of the colony was quite a time-consuming task to try and keep up with the coming-and-going of the adults, as well as the young which would soon

Annotated print-out of Sand Martin colony at Wenhaston Black Heath

scramble back down the nest holes at the first sign of danger – alerted by the calls of their ever- vigilant parents. incidentally, both Sparrowhawks (Accipiter nisus) and Hobbies (Falco subbuteo) were regular visitors to the colony, but usually the adults were incredibly quick at seeing them and sounding the alarm. i did not observe any predations during the summer. it was also a very rewarding experience and provided me with a huge insight into the amount of effort required by these birds to excavate the holes, incubate the young and then keep them fed. thankfully the summer was generally dry and fine; fortunately one wet spell in July didn’t seem to affect them adversely, although it coincided with the start of the second broods, which may well have been fortuitous.

Miscellaneous observations

Not surprisingly, the main diet of Sand Martins is flying insects, and usually pretty small ones at that. i was interested to see that one of the youngsters had been served up with a rather large crane fly (Tipulidae), which was eventually consumed after a bit of a struggle! it was evident that a small proportion of the youngsters had ticks around their faces, mostly in those holes that had the larger broods and invariably it seemed that the smaller, less-dominant birds were those with most ticks. No doubt the rather crowded and stuffy environment at the end of a one-metre long nest hole provide the ideal environment for bird ticks to survive. clearly these parasites must influence the longevity of those birds affected. the parents were very diligent at removing the faecal sacs from the end of

Sand Martin nest with five young (note the ticks on the left-hand bird)

the nest holes and it was rather comical watching them trying to scramble back into the nest hole whilst the youngsters were right at the entrance calling loudly for food. Quite frequently the young refused to budge and the adults had to literally scramble underneath them to get back into the hole!

it is well known that Sand Martins are rather prone to ‘dreads’ at nesting colonies and this happened quite frequently at Wenhaston. the typical alarm call of adults is a loud repetitive trill, usually uttered when they see a raptor approaching, mainly Hobby, Sparrowhawk or kestrel (Falco tinnunculus), although i did hear the same alarm call when a juvenile cuckoo (Cuculus canorus) flew in and landed at the top of the quarry! these alarm calls always resulted in a very swift dispersal of all the birds away from the nesting area, often in one flock, but sometimes in all directions; at the same time all of the juveniles amassing at the end of the hole waiting to be fed scurried rapidly back down into their holes. the adults frequently returned within a few minutes if the ‘coast was clear’, but it would take a lot longer before the juveniles would gather enough courage to creep back to the entrance of their holes.

Postscript

it was perhaps inevitable that 2021 would prove to be a disappointment compared with 2020, so a total of no more than nine nesting pairs is a bit of a comedown after the success of the previous year.

Acknowledgements

i should like to thank Alan Miller (recently retired from SWt) and Jonathan Alder (WcG) for encouraging me to write an account for the local village newsletter, which has prompted me to write this more extensive article, and also for providing me with a history of the management at this site.

Adult bird removing a faecal sac from a nest hole

References:

Balmer, D.e., Gillings, S., caffrey, B.J., Swann, r.L., Downie, i.S. & fuller r.J. 2013. Bird Atlas 201711: the breeding and wintering birds of Britain and Ireland. Bto Books, thetford. cramp, S. (ed.) 1988. Handbook of the birds of Europe, the Middle East and North Africa: the birds of the Western Palearctic. Vol. 5: tyrant flycatchers to thrushes. oxford university Press. Mead, c.J. 1979a. colony fidelity and interchange in the Sand Martin, Bird Study, 26:2, 99-106. Mead, c.J. 1979b. Mortality and causes of death in British Sand Martins. Bird Study 26:2, 107112. Mead, c.J. & Harrison, J.D. 1979. overseas movements of British and irish Sand Martins. Bird Study 26:2, 87-98 Piotrowski, S. 2003. The Birds of Suffolk. Helm, London. robinson, r.A., Balmer, D.e., and Marchant, J.H. 2008. Survival rates of hirundines in relation to British and African rainfall. Ringing & Migration 24: 1–6. turner, A. & rose, c. 1989. A Handbook to the Swallows and Martins of the World. Helm, London.

A juvenile bird trying to re-enter its nest hole and being mistaken for an adult by its fellow siblings!