Local Gull Movements as a Hazard to Aircraft

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1 Bird Study ISSN: (Print) (Online) Journal homepage: Local Gull Movements as a Hazard to Aircraft David R. Grant To cite this article: David R. Grant (1974) Local Gull Movements as a Hazard to Aircraft, Bird Study, 21:3, , DOI: / To link to this article: Published online: 23 Jun Submit your article to this journal Article views: 106 View related articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 19 February 2017, At: 00:49

2 Local Gull Movements as a Hazard to Aircraft by David R. Grant The increase in air traffic brings with it an evergrowing bird-strike hazard; the risks to life and property make this an urgent problem worthy of the closest investigation. The author, reporting on a study of gull movements around Edinburgh Airport, suggests that a useful 'bird forecast' could be based on a knowledge of the birds' daily and seasonal feeding and roosting habits. AT LEAST Two MAJOR bird-aircraft collisions have occurred at Turnhouse (Edinburgh) Airport in the past ten years, both potentially fatal. The first, in November 1962, involved a Vanguard taking off at night in bad weather with poor visibility. At 100 m. it struck what the pilot later described as a 'white wall of birds': two engines failed and a third had to be kept running beyond normal operational limits Only by extremely skilful flying was the pilot able to circle and land safely. None of the 76 people on board was injured, but the aircraft cost about 100,000 to repair: 125 gulls were later picked up dead on the runway. The second incident took place in January A twin-engined Dove aircraft collided with a flock of gulls which were fighting out to feed just after dawn. One engine failed completely but the pilot was able to return safely to the airfield. Again, over 100 dead gulls were found on the runway. A number of smaller incidents, mostly involving gulls, and sometimes causing minor damage, occurred during the same period. The problems encountered at Turnhouse arise almost entirely from the diurnal movements of Black-headed Gulls Larus ridibundus, Common Gulls L. canus and to a lesser extent Herring Gulls L. argentatus. References to 'gulls' in this paper are to mixed groups of these species, the usual ratio approximating to 30 : 40 : 1. The two species of blackbacked gull L. fuscus and L. marinus occurred infrequently and in negligible numbers. Gulls roosting on the Firth of Forth offshore from Cramond (see Figure 1) fly inland to feed each morning and back to roost each evening throughout the year. These movements cross directly over the airfield and create the major risk to aircraft when landing and taking off. Less frequent seasonal migratory flights and chance bad-weather movements are also a potential hazard, but are more difficult to evaluate from the point of view of air safety. 169

3 BIRD STUDY Figure 1. Turnhouse Airport and environs, showing the direction of flight taken by gulls fighting out from the Cramond roost to feed, their feeding area and main road, railway and river networks. 170

4 GULL MOVEMENTS AS A HAZARD TO AIRCRAFT The general nature of the problem of birds and aircraft is now wellknown and good accounts of it exist (e.g. Saul 1967, Stables and New 1968). The aim of this study was to examine the patterns of bird movement around Turnhouse and subsequently provide the airport authorities with information which might be of help in reducing the incidence of bird-strikes. Gull movements around Turnhouse cannot be considered independently of those in the Forth and Lothians as a whole. As a basis the annual cycle, distribution, growth and dispersal of local breeding populations of Black-headed and Herring Gulls were examined. The annual winter immigration of gulls from abroad and their departure in spring was also studied, as were the availability and distribution of winter food supplies in the area. Details of these aspects will be published elsewhere and will be mentioned here only when they bear on the Turnhouse problem. A full account can be found in Grant (1969). Observations and counts were made from the aircraft control tower at Turnhouse, supplemented by counts along the coast from South Queensferry to Seafield (see Figure 1). These counts were made once weekly as far as possible, normally covering a 24-hour period from Thursday afternoon till Friday at the airfield, followed by observations at the coast. Less frequently, counts were made inland, but as the gulls were widely scattered when feeding these were unreliable. Because of the overlap in the timing of breeding and migration between different species, as well as between British and foreign gull populations, the year had to be divided arbitrarily as follows : Period A-15 March-19 July Corresponds to time of assembly at colonies by British breeding gulls. Period March and 20 July-31 Oct. Corresponds to main passage of spring and autumn migrants and to dispersal of young and adults from British colonies. Period C-1 Nov.-28 Feb. Corresponds to the time of occupation of wintering areas. Even within these divisions overlap occurs, especially between the departure of winter immigrants and the nesting of British birds. All times given are in hours GMT. THE OBSERVATIONS Intensity of gull movements over Turnhouse The actual number of gulls observed passing over Turnhouse during the study period is shown in Figure 2. Insufficient observations were made in November and December for inclusion in the figure but the pattern for these months probably differs little from that of January and February. Both the number of birds involved and the timing of their movements varied according to season and day-length. Numbers were greater and took less time to pass over in winter than in summer. Also, as expected from the shorter days in winter, movements then took place later in the morning and earlier in the evening than in summer. 171

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6 GULL MOVEMENTS AS A HAZARD TO AIRCRAFT Direction of movement and hourly passage of gulls flying over Turnhouse lithe information was to be of use to pilots and air-traffic controllers, it was obviously desirable to represent gull movements more precisely than the broad picture given in Figure 2. Figure 3 shows the direction of gull movements during the three arbitrary periods of the year expressed as a rate per hour of gulls flying over the airfield. The direction of movement was largely south in the morning and north in the evening, irrespective of period. The highest rate of movement was in winter (C), the lowest in summer (A), with spring and autumn (B) intermediate. Remembering that the morning movement was usually observed on the day following the evening one, there was a remarkably close agreement between the morning and evening rate within each period. Tide and weather factors, which in some instances varied considerably from the evening to the following morning, probably caused some of the discrepancies, and if the observations had been made on the same day, even closer agreement might have been obtained. The direction of movements during periods A and B included a small proportion which went to all points of the compass (except west in B; evening), whereas during period C this was not so, suggesting that in winter the movements are more directed than at other times of year. No apparent difference was found between spring and autumn directions of movement. Although Figure 3 seems to show more birds going north than south in period B and more south than north in period C, this was probably due to the observations being made on different days. Any migratory movements which might have accounted for these variations were unlikely to have been associated with feeding and roosting flights. Observed migration was always distinct from local movements, mainly by timing. Observations between 1000 and 1500 hours were made less often than at other times and may not therefore be entirely reliable. They do indicate that movement is drastically curtailed during the middle of the day at all seasons, though the evening flight starts earlier than 1500 hours in winter. The rate per hour of gulls flying over Turnhouse Figure 2 gives a general idea of the overall pattern of gull numbers at Turnhouse throughout the year. In Figure 4 the data have been divided into periods and expressed as a rate per hour. In period A the rate at which gulls passed never exceeded 70 per hour and was usually considerably lower. Small peaks in movement occurred between 0500 and 0700 hours and between 1500 and 1800 hours, with a second evening peak at 2000 hours, but numbers were never very great. The peak at 2000 hours was caused mainly by a flock of immature Common Gulls which rested on many days in a young potato crop adjacent to the airfield and departed to roost as a flock about this time. It was not therefore a strictly typical movement. 173

7 BIRD STUDY During period B the morning rate was over 1,350 gulls per hour at its maximum, while the evening maximum was 1,230 gulls per hour. Peak rates were found between 0400 and 0600 hours in the morning and between 1500 and 1800 hours in the evening. The longer duration of the evening movement accounts for the maximum rate being lower than in the morning. The pattern in period C was similar to that of period B, but with greater numbers involved. The maximum rates were 2,048 gulls per hour in the morning and 1,166 in the evening. The morning movement was at its greatest for two hours from 0700, and the evening movement for nearly four hours from If the maximum morning rate is multiplied by two (hours) and the maximum evening rate by four (hours) the products are 4,816 and 4,664 respectively. Thus approximately the same number of gulls passed over at both times. GULLS Figure HOURS G M T 4. The rate per hour at which gulls flew over Turnhouse Airport. Period A, Period B, Period C, 174

8 GULL MOVEMENTS AS A HAZARD TO AIRCRAFT The number per hour of gulls sitting on the airfield As well as birds on the airfield these counts included gulls sitting in surrounding fields and close enough to be considered a potential hazard. More were present in the mornings and evenings than during the middle of the day. This was due to several factors. First, in the morning, gulls fighting inland sometimes landed on or near the airfield initially, only leaving when disturbed by people moving about. This was especially true of days with long hours of light when movement inland began well before most people were on the move. Gulls which landed on the airfield itself were usually scared off by airport staff as a matter of routine, and often left the vicinity altogether. Both forms of disturbance resulted in a drop in numbers in mid-morning. Secondly, agricultural activities influenced the number of gulls present. Ploughing, harrowing, disking and hay-turning operations all attracted gulls. When, for example, ploughing was started in a nearby field gulls would come into the area fairly quickly, even though very few may have been present when the ploughing began. Thirdly, weather, tidal and seasonal factors may also have influenced whether or not gulls were present during the day (see below). All these factors, or combinations of them, led to tremendous variation in the number of gulls to be seen in the area at any one time. Nevertheless, morning and evening peaks were discernible. The evening peaks were caused largely by gulls, which were moving back to the coast, landing to feed for a short time en route. Very often the airfield and surrounding land was undisturbed in the evenings. In short there is no one simple explanation for the fluctuations found in the number of sitting gulls. There is an exception to this pattern which, because of its potentially high danger, requires explanation. On several occasions in late August, between 0200 and 0600 hours, very large flocks of mainly Common Gulls appeared on the airfield, perhaps because its undisturbed expanse of grass presented an attraction. Twice flocks in excess of 2,000 birds were seen and on at least three other occasions vast amounts of droppings and feathers were found on the runway intersection. These flocks were almost certainly newly arrived immigrants, possibly making their first landfall. Turnhouse is normally closed from 0030 to 0630 hours so there was no threat to aircraft from these flocks. However, if the airfield should ever become operational for a full 24-hour period then they could become a serious hazard, especially as it would be difficult to forecast their arrival accurately. Late August and early September is the time to expect them. Height of flight The heights at which gulls were flying over Turnhouse were noted, when possible, by reference to known heights of nearby buildings. Although subject to observer bias all the observations were made by the writer and should at least be reasonably consistent. A total of 26 observations were made in the morning and 22 in the evening. 175

9 BIRD STUDY Chi-squared tests showed that the morning flight in winter took place at a significantly lower altitude than the evening one (P=0.01), but no such difference was found in summer The gulls also flew higher in winter than in summer, e.g: 82% flew at over 100 m. in winter but only 40% did so in summer Similarly, 35.6% flew at over 200 m. in winter while only 0.4% did so in summer The difference is due partly to thermal soaring by gulls on winter evenings. Wind direction in relation to height of flight was not examined in detail but gulls fly extremely low on days with exceptionally high winds. Thus on 30 January 1969, when the mean wind-speed was 22 knots, with gusts up to 53 knots, the morning flight took place between 3-7 m. above ground, the gulls following the contours in long streams rather than flying high on a broad front as usual. Factors affecting local gull movements Several factors influenced the number of gulls seen at Turnhouse. (a) Weather. Observation led to the conclusion that weather played a considerable part in determining the number of gulls to be found on the airfield. When the ground was wet, especially if standing water was present, then the airfield was more attractive to gulls than when it was dry. Temperature had little effect except when it produced hard frost, which made it impossible for the gulls to probe for food. Windspeed only became important when it was exceptionally high, when it may have deterred gulls from utilising the exposed area formed by the airfield. Seasonal effect has already been demonstrated. Because of great variability, it was not possible to show whether these factors were statistically significant. (b) Tide. Gulls which flew over Turnhouse were mostly commuting from tidal mud-flats (Drum Sands) at the mouth of the River Almond at Cramond. The area is normally used by up to 20,000 gulls each night but when the mud-flats are uncovered large numbers also feed there by day. If the mud-flats were uncovered when the gulls were due to leave the roost and fly inland, then a proportion at least could be expected to remain. It might also be expected that they would feed there at other times of day provided that the mud was exposed. Testing the interaction between tidelevel and time of day using an analysis of co-variation it was found that low tides in the morning or evening have a statistically significant (P = 0.05) effect on the number of gulls which pass over Turnhouse. More gulls feed at the coast at low tide and therefore fewer fly over Turnhouse in the morning and evening than at times of high tide. Crook (1953) suggested 'very tentatively' that such a situation might be true of gulls at Southampton Water. In fact this is probably a widespread feature of gull movement at all places where similar tidal situations occur at roosts. The nature of local gull movements and feeding behaviour At least in winter, when gulls form the greatest hazard to aircraft at Turnhouse, their movements are remarkably regular within the limits imposed by season, weather and tide.to make prediction of these movements as accurate as possible it is also necessary to take into account their nature and consider what governs their apparent regularity. 176

10 GULL MOVEMENTS AS A HAZARD TO AIRCRAFT Winter movement is almost entirely connected with fighting between roosting and feeding areas, and this occurs soon after dawn and just before dusk. The exceptions are hard weather movements which result in a temporary sojourn on the coast by birds roosting on inland waters while their roosts are frozen up. At such times inland feeding ceases almost entirely apart from refuse-dumps, because frozen ground renders food inaccessible. Permanent food sources such as rubbish-dumps and sewer-outlets attract mainly Herring Gulls. Birds are present at these throughout the day in winter and to a lesser extent in summer. In frosty weather attendance at these food sources increases rapidly and greatly, Common and Black-headed Gulls making up a much higher proportion of the total then than at other times. Inland, Black-headed Gulls are usually found wherever agricultural operations are disturbing the ground. Common Gulls show a preference for short grassy fields but also follow the plough frequently. Both species take advantage of chance food supplies such as picnic remains; but for the most part, once they have found a food supply, they return to it daily until it is exhausted or ceases to be available. Whether flock continuity persists from day to day at any one place is not known, but both Crook (1953) and Hickling (1957) thought this unlikely. An experiment with 100 colour-marked Herring Gulls caught at Hailes Quarry rubbishdump in Edinburgh showed that at least a proportion (up to 10%) returned on subsequent days. However, 100 birds represented only 2%- 3% of the total usually found at the dump, and only a small fraction of the total in the Edinburgh area. The important point from the aircraft operators' view is that gulls do tend to return daily to the same feeding places, numbers varying according to the factors mentioned. Use of radar to locate bird movements After the study was completed, a Plessey AR-1 radar set was installed at Turnhouse, and following a request in April 1970 an investigation was made during the set's pre-commissioning trials into some unidentified echoes which were appearing intermittently on the screen. These were bird echoes, chiefly from gulls, whose movements corresponded precisely with those seen visually during the study. A report on the interpretation of these echoes was submitted to the Air Traffic Control Evaluation Unit, which was carrying out the commissioning, and was subsequently included as an appendix to their own report (Ministry of Defence 1970). By using this radar, with even limited knowledge of the local gull and other bird movements, aircraft approaching Turnhouse could be warned of possible danger from large bird-flocks in the vicinity. It might also be possible with experience to route aircraft clear of large transitory flocks. This could be especially important when the proposed new runway becomes operational, as it is planned to lie on a line which will bring the flight path of aircraft taking off or landing almost directly over the Cramond gull roost. 177

11 BIRD STUDY DISCUSSION AND CONCLUSIONS The bird hazard at Turnhouse is mainly caused by the winter feeding and roosting flights of three species of gulls. These movements are all predictable. Gunn and Solman (1968) have already suggested that a bird forecast, on the lines of a weather forecast, could become a daily routine at airfields. Given the known position of bird movements at Turnhouse this could now become a reality. The cost of the Vanguard incident alone would have been sufficient to pay the salary at present levels of a bird forecaster for over 50 years! In fact, forecasts of movements of large birds have already been introduced in Europe. The logical progression from this is surely local bird forecasting. More information is needed about the irregular movements which take place in autumn and winter under conditions of extremely low cloud with high winds and persistent heavy rain. They may involve migrant birds, which have become disoriented, or more likely birds which normally roost at the coast and are seeking shelter inland because their roost is untenable in such conditions. Such flocks can often be the worst hazard and one was probably responsible for the Vanguard incident in There is no certain remedy, only extra vigilance on the part of air traffic control staff in keeping the runways clear prior to aircraft taking off or landing. The use of radar to scan for such flocks could also prove of great value in these circumstances. Whether it would be possible to clear the main source of birds causing a hazard at Turnhouse, the extensive gull roost at Cramond, is difficult to know. It has been established for a long time and is also important as a feeding area, not only for other gulls but also for large numbers of ducks and waders. Any attempt to clear it would have to be made only after very carefully considered planning, taking into account not only where the birds might go but the effect on other species as well. If such an operation were feasible it should certainly go a long way towards reducing the bird hazard at Turnhouse. The prime purpose of this study was to provide information to the airport authorities on the bird hazard at Turnhouse. Recommendations based on the results obtained have been incorporated in instructions to personnel concerned with bird problems at the airport. Providing the problem is always kept under review and changes which may occur in the pattern of movements in the future are noted, it is hoped that further serious bird strikes there may be prevented. ACKNOWLEDGEMENTS Too many people provided assistance in this study to mention them all by name However, I must express special thanks to G. M. Mackintosh and the (then) Board of Trade for permission to work at Turnhouse; to the Air Traffic Control Staff for their co-operation and to Dr J. D. Lockie and Dr D. H. Mills of the Department of Forestry and Natural Resources, University of Edinburgh, who 178

12 GULL MOVEMENTS AS A HAZARD TO AIRCRAFT supervised and encouraged me throughout. Great help was also received from J. Scott of Edinburgh Corporation Cleansing Department, C. D. T. Minton of The British Trust for Ornithology, and T. Brough and C. Bridgman. Dr J. D. Lockie (Edinburgh University), Dr I. Newton (Nature Conservancy) and Dr W. R. P. Bourne (Aberdeen University) gave valuable criticism of the draft. SUMMARY Gulls fighting inland to feed from a roost on the south shore of the Firth of Forth present a persistent hazard to aircraft at Turnhouse (Edinburgh) Airport. One major bird-strike has occurred during morning flight, and a second when a flock of gulls which had apparently become disoriented at night in bad weather was hit by an aircraft taking off. It was found that gulls regularly flight over the airfield at dawn and return at dusk, though the timing and scale of the movements varies seasonally and may be modified by the weather and the tidal cycle at the coast. The movements can be seen with radar and a daily bird-forecast could become routine, so that pilots could be warned to avoid the birds. Whether the situation could be improved by removing the main hazard, the coastal roost in the Firth of Forth, is problematical. REFERENCES atom s. H An observational study of the gulls of Southampton Water. Brit. Birds, 46 : GRANT, I/ R The dispersion of gulls in the Lothians. Unpublished M.Sc.Thesis, University of Edinburgh. GUNN, W. W. H. and V. E. P. SOLMAN A bird warning system for aircraft in flight. In The Problem of Birds as Pests, Institute of Biology Symposium No. 17, Academic Press, London. HICKLING, R. A The social behaviour of gulls wintering inland. Bird Study 4: MINISTRY OF DEFENCE Flight Trials of the Plessey AR-1 Radar at Edinburgh (Turnhouse) Airport. ACTEU Rep. No SAUL, E. K Birds and aircraft : a problem at Auckland's new international airport. J. Roy. Aeronaut. Soc., 71 : STABLES, E. R. and N. D. NEW Birds and aircraft : the problems. In The Problems of Birds as Pests, Institute of Biology Symposium No. 17. Academic Press, London. D. R. Grant, Department of Natural Resources, University of Edinburgh. (Present address: Hawkslee Farm Animal Boarding Centre, St Boswells, Roxburghshire). 179