Volume 68 Number 3 March 1975

Birds and other animals at Sand Martin colonies C. J. Mead and G. R. M. Pepler

INTRODUCTION One of the traditional features of the 'Notes' section of this journal has been to publish data on nesting associations. In recent years several notes have appeared concerning species breeding in the holes of Sand Martins Riparia riparia; the most recent were by Durman (1973) and Mather (1973b). During the 1960's we were both involved in catching and ringing Sand Martins during the breeding season as part of the British Trust for Ornithology's ringing enquiry into the species. We often noticed nesting associa­ tions with the Sand Martins, and also witnessed predation on them and on their ectoparasites. This paper summarises our observations. Many other ringers also involved with the enquiry must have collected similar information from other parts of the country, but no attempt has been made to obtain these data from them.

COVERAGE AND METHODS The geographical extent of our combined experience at Sand Martin colonies can best be gauged from fig. 1, which shows the estimated number of colony visits in each county during 1960-69 (the old counties are used throughout this paper). In all nine counties with more than ten visits, at least one colony was visited regularly over a period of years. Between us, we were probably present at more than 50,000 'bird handlings' during several hundred visits to well over 100 different colonies.

[Brit. Birds, 68: 89-99, March 1975] 89 90 Birds and animals at Sand Martin colonies

Fig. I. Counties where the authors visited colonies of Sand Martins Riparia riparia for ringing during 1960-69. Dot sizes indicate the estimated number of colony visits in each county

(The number of visits in Berkshire should be shown in the 10-24 category, not 25-99)

Most of the colonies were in man-made situations, generally actively worked sand quarries, and, except in Scotland and Wales, hardly any were discovered in 'natural' sites. The normal ringing practice was to erect mist-nets in front of the holes at night to catch the birds emerging in the morning. Extensive experiments before the start of the national enquiry had shown that this technique was most effective and that, provided that the birds had not been dis­ turbed during the excavation phase of the breeding cycle (generally completed during the first part of May), it did not cause the Sand Martins to desert. The nets were removed from the holes while the first catch was ringed and recorded, and were then re-erected for a second catch. These techniques allowed birds of any species occupy­ ing Sand Martin holes to be caught and examined, and also meant Birds and animals at Sand Martin colonies 91 that the ringers were present at, or very near, the nesting burrows for a considerable part of the time spent at the colonies.

NESTING ASSOCIATIONS The most obvious nesting association involved another species breeding within an occupied colony in unmodified Sand Martin bur­ rows. In the case of species which actively excavate their own bur­ row, nothing short of continuous observation could prove whether or not the hole being used had been dug initially by a martin. The systematic list that follows does contain examples of perfect burrows being used by other species, but several instances refer to imperfect, relict holes being utilised by rather unlikely birds. In all cases, since the actual nesting chamber of the Sand Martin, whether still at the end of a burrow or exposed by erosion, was probably being used by the intrusive species, it would be quite possible for parasites normally associated with the martins (see page 94) to be transferred to other hosts. For instance, the tick Ixodes lividus has been taken from Tree Sparrows Passer montanus occupying Sand Martin burrows (S. F. Barnett in litt.). We have not conducted a search of the literature, which contains many records of nesting associations (for example, The Handbook included 'Sand Martin's burrow' among recorded nest sites of the Wryneck Jynx torquilla); all are from our personal experience. Perhaps one exceptional species might be mentioned, though our only direct experience of its nesting with Sand Martins was at Zaragoza, Spain: the Bee-eater Merops apiaster. The 1955 nesting record from Sussex (Barham et al. 1956) was from a Sand Martin colony at which one of us subsequently ringed. In most cases, the list that follows gives the number of colonies within each county where the species in question was recorded (one colony where no number is specified). In many instances, several pairs of the intrusive species were present in a single colony; also, species were often present for several years. Kestrel Falco tinnunculus: Norfolk (two); Dorset, Hampshire and Suffolk. In all cases the nests were in greatly enlarged holes or rifts in the colony face. In two instances the enlargements were probably caused by children playing, but the other nests were in holes either enlarged by the effects of the weather or, possibly, caused by animals before the pit was excavated. The nests were all situated well into the face; it is doubtful that an exposed Kestrel nest in an active colony could be successful, as the occupants would be mobbed continuously by the martins as long as they were in sight. Stock Dove Columba oenas: Hampshire, Norfolk and Suffolk (several in each); Bedfordshire (three); Berkshire, Dorset and 92 Birds and animals at Sand Martin colonies Oxfordshire (two each); Cambridgeshire, Huntingdonshire and Sussex. This species was often present in surprisingly narrow holes within the main part of a colony. Sometimes several pairs were nesting at one site. One Suffolk colony had four pairs in the face occupied by Sand Martins and at least a further four down burrows of Rabbits Oryctolagus cuniculus, all active at the same time. Little Owl Athene noctua: Hampshire (two); Bedfordshire and Norfolk. In all cases the occupied hole was at the edge of the colony. Although one of the Hampshire holes had probably been enlarged by human digging, the others were probably 'natural'. Kingfisher Alcedo atthis: Berkshire and Hampshire. In both cases the presence of a Kingfisher nest at the colony was probably fortuitous, the conditions for one species to be able to dig its burrow being suitable also for the other. One nest was directly over water, and suitable streams for the birds to fish in were within easy reach. Jackdaw Corvus monedula: Hampshire (two); Berkshire, Cambridge­ shire, Dorset, Norfolk, Perthshire and Suffolk. Large holes at old colonies were often regularly occupied from year to year. At the Norfolk colony four or five pairs were sometimes present. Great Tit Parus major: Hampshire (four); Norfolk and Suffolk (three each); Bedfordshire (two); Dorset, Fife, Nairnshire and Sussex. Great Tits can easily enter and use normal Sand Martin holes for nesting. All the occupied holes were at colonies with trees close at hand. Blue Tit Parus caeruleus: Oxfordshire (four); Hampshire (three); Lincolnshire and Norfolk (two each); Bedfordshire, Berkshire, Perthshire and Sussex. Even the narrowest Sand Martin hole can be used. Several occupied holes were a considerable distance from the nearest trees. Coal Tit Parus ater: Aberdeenshire, Inverness-shire, Norfolk, Perthshire and Suffolk. It is rather surprising that more Coal Tit nests were not found, since sand outcrops are often associated with coniferous trees and Coal Tits frequently nest in holes in the ground, among tree roots or, when they are available, in stone walls. Wren Troglodytes troglodytes: Dorset and Hampshire. In both cases the nesting chamber of a partly eroded burrow was used as a base for a normal domed Wren's nest. Both were at colonies in old sand- faces partly overhung by dead vegetation and roots. Blackbird Turdus merula: Oxfordshire. The exposed nesting cham­ ber at a badly eroded face provided a platform for a typical Black­ bird nest. Birds and animals at Sand Martin colonies 93 Robin Erithacus nubecula: Bedfordshire. The nest was constructed in the same situation as the two Wrens' nests described above. Spotted Flycatcher Muscicapa striata: Suffolk. The nest was built in the exposed nesting chamber of a Sand Martin burrow; although partly concealed by overhanging vegetation, it was not supported by vegetation or roots. Pied Wagtail Motacilla alba: Berkshire, Huntingdonshire, Norfolk and Oxfordshire. In all cases a partly eroded hole was used as the base for a nest. In the first two counties the nests were close to the lip of the face and among the roots of the overlying vegetation, but in the others they were partly inside crumbling holes on a sand- face free of vegetation. Starling Sturnus vulgaris Almost every well-established colony had Sand Martin holes taken over by Starlings. The only county of the 30 visited (see fig 1) where they were not found was Kincardineshire, where only one colony was visited. In our experience newly dug holes were never taken over by Starlings, as their breeding season starts so much earlier than the Sand Martins. However, holes from previous years were available; if the sand-face had survived the winter, at least one burrow was usually taken over by Starlings. Some colonies housed up to a dozen pairs. House Sparrow Passer domesticus Again, most well-established colonies had some holes occupied by House Sparrows, but a few, remote from human habitation, had none. No House Sparrows were present in the few colonies we visited in Carmarthenshire, Kin­ cardineshire, Midlothian and Morayshire. This pugnacious species is probably able to dispossess Sand Martins but, as with the Starling, most pairs are likely to be 'in residence' before the martins are showing serious interest in the colonies. Their presence provided apparent amusement for the newly fledged Sand Martins. While still within the colony, the young martins, frequently in large gangs, very often chased drifting feathers, many of which were provided by the untidy nests of the sparrows. Sometimes adult sparrows, while attempting to add material to their nests, were so harried by the mobs of martins that they were forced to drop new-found feathers to the martins' evident delight. Tree Sparrow Passer montanus Although this species is not nearly so widely distributed as the House Sparrow, we found it nesting in most of the Sand Martin colonies within its range. All counties where we caught Sand Martins between the Thames and the Hum- ber had colonies with Tree Sparrows, as did Sussex. Some contained ten or more active nests, and even colonies remote from wooded areas did not escape this species' attention. 94 Birds and animals at Sand Martin colonies Other hirundines Although not a nesting association as such, the two other British breeding hirundine species are occasionally found at Sand Martin colonies during the nesting season. Mather (1973a) mentioned a record of a juvenile House Martin Delichon urbica entering a hole in an occupied colony, possibly to roost. Up to one in every thousand of the martins we trapped at Sand Martin colonies were actually House Martins. Although these were often attracted to the colony simply by the activity of the Sand Martins, there were several instances when House Martins were caught in the first morning catch and it was thus inferred that the individuals concerned had spent the night in a burrow. This also happened, though much less frequently, with Swallows Hirundo rustica.

PREDATION AT SAND MARTIN COLONIES We have recorded several species of birds and mammals taking or attempting to take Sand Martins from burrows or while flying, apparently feeding on parasites at die nest entrances, or scavenging below the occupied face. All such activities are described in the following sections. The parasites taken, familiar to all Sand Martin ringers, were almost certainly the flea Ceratophyllus styx which overwinters in Sand Martin burrows and forms a 'reception committee' (Rothschild and Clay 1952) for the returning migrants. The fleas are stirred into action by the first visit of a bird to the burrow where they have overwintered. If the weather is warm enough, they migrate down the burrow to sit clustered on its lip, ready to jump on to anything which passes close enough to put them in a dark shadow. At this stage they are readily visible and can be pecked off by any bird willing to brave the inevitable shower of fleas provoked by its own shadow.

Avian predators Buzzard Buteo buteo At a Dorset colony a Buzzard landed on the lip of a sand-face occupied by Sand Martins and started to eat what proved to be a freshly dead juvenile. Later, what was assumed to be the same Buzzard spent some time apparently trying to take young Sand Martins which were congregating on the sand-face; it was persistently mobbed by a cloud of martins. It is not possible to say with complete certainty that the bird was starting to eat a Sand Martin it had itself killed rather than a corpse it had found. The body was, from the state of its primaries, of a fully fledged bird. Sparrowhawk Accipiter nisus Although Sparrowhawks were at a low ebb during the period we were trapping Sand Martins, they Birds and animals at Sand Martin colonies 95 were undoubtedly regular predators at colonies in districts where they were still present. The hunting hawk normally appeared low and fast over the lip of the occupied face of the pit and took a martin, in flight, before any of the birds realised what was happening. Passing, cruising Sparrowhawks and those flying slowly having made a kill were persistently mobbed by the Sand Martins. Records of predation by this species came from several colonies in Hampshire, from two each in Dorset and Norfolk, and from one each in Bedford­ shire, Berkshire, Nairnshire, Perthshire and Suffolk. Hobby Falco subbuteo This species was a common predator at colonies in southern England. The Hobby would 'wait on' over the colony and suddenly stoop very fast, 'throwing up' at the end to take a martin. During the quiet of an early morning, the sound of the Hobby rushing through the air followed by the distinct thud of its impact on the unfortunate martin was unforgettable. Some large colonies which were obviously close to nesting Hobbies were sub­ jected to very considerable predation. During a late July visit to one Hampshire colony, lasting six hours, there were eleven Hobby kills (every stoop successful). Almost every colony regularly worked in Hampshire and Dorset was subjected to Hobby predation at some time during our visits. It was also recorded in Bedfordshire, Berk­ shire, Norfolk, Suffolk and Surrey. Several autumn roosts of Sand Martins which we visited were also subject to Hobby predation. In some cases locally breeding Hobbies (or off-passage birds) appeared every evening at the time that the martins started to gather. At Chichester, Sussex, up to three Hobbies were present together in some years. Merlin Falco columbarius At an Inverness-shire colony a Sand Martin was taken in flight by a Merlin which appeared from, and returned to, adjoining hill country. The ease with which it took the martin seemed to indicate that it regularly fed on them. Kestrel Falco tinnunculus A ubiquitous predator at Sand Martin colonies, this species was very much less successful than the Hobby in taking flying martins. All raptors are mobbed by Sand Martins, which have a special alarm call to warn of the presence of avian predators. Successful Kestrel predation most often took place close to the face of the colony and presumably most often involved young birds. Sand Martins caught in mist-nets proved particularly attrac­ tive and this resulted in our capturing and ringing over 30 Kestrels, compared with only a handful of Sparrowhawks and a single Hobby. Prairie Falcon Falco mexicanus A free-living Prairie Falcon, later discovered to have been lost six months previously, took a Sand Martin at a Bedfordshire colony in a most spectacular fashion. 96 Birds and animals at Sand Martin colonies Black-headed Gull Lams ridibundus At several colonies in Hamp­ shire and at one in Dorset this species was regularly seen coming down to hole entrances. The gulls' actions were usually consistent with flea feeding, but they readily took the contents of exposed nests after cliff falls. Barn Owl Tyto alba At two colonies, in Hampshire and Norfolk, Barn Owls came over in daylight and swooped down at the occupied face. At the Norfolk colony a bird (presumably a Sand Martin, though Tree Sparrows were also present) was taken. Little Owl Athene noctua At one of the Hampshire colonies where Little Owls were breeding (see page 92), both birds of the pair attempted to catch Sand Martins during the morning. One martin was taken, and it is likely that young, free-flying martins are vulner­ able to this species. Tawny Owl Strix aluco At a Hampshire colony a Tawny Owl appeared several times at the occupied face and attempted to catch Sand Martins. At least one attempt was successful (only martins were present). All three owl species were seen attempting to catch Sand Martins only during the later part of the season when young, inexperienced birds, not yet capable of expert flight, were available. Corvids All five widely distributed species were seen to show interest in Sand Martin holes, apparently as a source of food. Jackdaws and Carrion Crows Corvus corone were definitely seen to peck at the entrances as if taking small items of food, presumably the fleas. Adult Rooks C.frugilegus were seen doing this only once, at a colony in Bedfordshire with Jackdaws. Carrion Crows, Jackdaws, Magpies Pica pica and, once in Lincolnshire, a Jay Garrulus glandarius were seen to take advantage of cliff falls both to predate exposed nests and also to scavenge Sand Martin casualties below the face. Magpies and, to a lesser extent, Carrion Crows were seen enlarging holes in obvious attempts to expose the nest contents. Only once were Rooks seen to do this.

Mammalian predators Mammal predation of, or at least interest in, Sand Martin nests was mostly inferred from tracks found in the sand at the burrow en­ trances. It was difficult to be certain what the mammals were doing, though pulled out nests were clear evidence of predation. Unlike avian predators, mammals cannot normally reach the entrances of burrows in colonies built in vertical faces. Mammal predation can thus take place only in the following circumstances: (1) Where the colony has been built on a low or sloping bank affording access to mammals. This is fairly frequent when, at a previously successful site, the birds are not afforded the opportunity, Birds and animals at Sand Martin colonies 97 by human activity or natural cliff falls, of building their burrows into a clean, vertical face. (2) Where erosion or human interference (usually quarrying operations but sometimes children playing) has created a scree reaching the holes or a ramp against the face. (3) Where the martins have built their holes at the top of the sand-face. Here predators may be able to climb down from the top using overhanging vegetation and roots. It would be possible for them also to dig out nests near the surface, but we have never had a clear indication that this strategy has been adopted—except by the young of Homo sapiens!

Hedgehog Erinaceus europaeus A Hedgehog was discovered at night below an occupied face in Hampshire eating the remains of a Sand Martin which had almost certainly been killed by a cliff fall. Red Squirrel Sciurus vulgaris In Norfolk a Red Squirrel was dis­ turbed from part of a colony behind overhanging tree roots. Marks at the entrance to a burrow behind the roots indicated that the squirrel had been trying to enter. This attempt was not successful, but this species is known to eat the eggs and young of birds (Southern 1964). Brown Rat Rattus norvegicus Brown Rats were invariably present at colonies close to pits used for tipping rubbish. Their characteristic tracks were seen several times on scree and at the entrances of low holes in occupied colonies. Some burrows had tufts of nest material visible and had obviously been entered and disturbed. Fox Vulpes vulpes The tracks of foraging Foxes were often seen in the sand at colonies and, where burrows were within reach, deter­ mined and often successful attempts were made to enlarge them and devour the contents of the nests. Stoat Mustela erminea and Weasel M. nivalis Mustelid tracks were sometimes seen at occupied colonies and both species would un­ doubtedly relish eggs, young or adult Sand Martins. The only direct evidence of successful predation was by a Stoat at a Norfolk colony. Badger Meles meles At two English colonies, in Hampshire and Suffolk, burrows which had been dug out in low banks showed the unmistakable prints of this species. At a Scottish colony, in Nairn­ shire, a Badger was disturbed as it approached an occupied sand- face, but no evidence could be found to suggest that any previous attempts to excavate burrows had been made.

Domestic animals, bats and invertebrates Cats and dogs have also been seen to interfere with Sand Martins at colonies near human habitation. Cats were never seen to attempt 98 Birds and animals at Sand Martin colonies to dig out birds from holes, but they often tried to catch both flying and perched martins. They were often successful in catching even the flying birds near the colony where the martins habitually flew very low. The cats normally concealed themselves in some sort of cover, sometimes up to a metre from ground level, to pounce on passing birds. The corpses examined were almost always of juveniles. Dogs generally paid little attention to the Sand Martins, but low holes were sometimes attacked. At one memorable colony the startled ringer discovered, at about midnight and by the light of a fast-failing torch, enormous dog footprints along the sand-face. It is at such times that warning notices about guard-dogs on patrol have their maximum effect! It is, perhaps, surprising that we have no records of bats (Chiro- ptera) entering or leaving Sand Martin holes. Several species com­ monly occurred in areas where we operated, but it seems likely that they were living in.buildings, hollow trees or other conventional sites rather than Sand Martin burrows. Obviously many invertebrates, apart from the parasites already mentioned, use burrows, but the only really memorable one was a species of wasp (probably Vespa sp) which had an active nest in an old hole at the edge of an occupied colony in Hampshire.

REMARKS From this summary it may seem that all sorts of disasters can over­ take nesting Sand Martins and that they are the gullible victims both of species which wish to usurp their nests and also of predators. This is, of course, not the case. Only a tiny proportion of the nests at a normal colony is interfered with by other species of birds and, usually, none is accessible to mammalian predators. The recent sharp decline of the species in this country, which coincided with the crash in numbers of Whitethroats Sylvia communis, was certainly not due to this sort of interference with the breeding cycle. Recent evidence (Winstanley et al. 1974) correlates the decline of the Whitethroat with climatic change south of the Sahara and this is most probably the case with Sand Martins also.

SUMMARY During several hundred visits to colonies of Sand Martins Riparia riparia for ringing purposes in the 1960's, 16 other species of birds were discovered nesting in Sand Martin burrows. Three species—Starling Sturnus vulgaris, House Sparrow Passer domesticus and Tree Sparrow P. montanus—were particularly often encountered using unmodified burrows, but several others utilised only eroded, enlarged or relict holes. The records are listed by species and enumerated by county. As well as these nesting associations, predation on Sand Martins (or scavenging) at colonies by six species of raptors, three owls and five corvids, as well as by Black-headed Gulls Larus ridibundus and six or seven different mammals, is also described. These records too are listed by species and enumerated by county, Birds and animals at Sand Martin colonies 99 and the strategy employed by the predator is mentioned. Sand Martin colonies in clean, vertical sand-faces cannot be reached by mammalian predators, but raptors may take quite a lot of birds at some colonies; Hobbies Falco subbuteo, at some sites in southern England, could well take hundreds of birds from a single large colony each year. Nevertheless, such predation is considered to be almost insignificant in controlling the total population.

REFERENCES BARHAM, K. E. I., CONDER, P. J., and FERGUSON-LEES, I. J. 1956. 'Bee-eaters nesting in Britain, 1955'. Bird Motes, 27: 34-43. DURMAN, R. 1973. 'Tree Sparrows and other species nesting in Sand Martin colonies'. Brit. Birds, 66: 450-451. MATHER, J. R. 1973a. 'House Martin apparently going to roost in Sand Martin colony'. Brit. Birds, 66: 447-448. 1973b. 'Tree Sparrows and other species nesting in Sand Martin colonies'. Brit. Birds, 66: 451. ROTHSCHILD, M., and CLAY, T. 1952. Fleas, Flukes and Cuckoos. London. SOUTHERN, H. N. 1964. The Handbook of British Mammals. Oxford. WINSTANLEY, D., SPENCER, R., and WILLIAMSON, K. 1974. 'Where have all the Whitethroats gone?' Bird Study, ai: 1-14.

C. J. Mead, British Trust for Ornithology, Beech Grove, Tring, Hertford• shire HP23 5NR G. R. M. Pepler, Stantonbury Education and Leisure Campus, Stantonbury, Milton Keynes, Buckinghamshire