Behavioural responses of herring gulls Larus argentatus to aircraft noise

Enrironmental P~lhttion(SeriesA) 24 (1981) 177 184

BEHAVIOURAL RESPONSES OF HERRING GULLS L A R U S A R G E N T A TUS TO AIRCRAFT NOISE

JOANNA BURGER

Biology Department, Livingston College, Center for Coastal and Environmental Studies, Rutgers University, New Brunswick. New Jersey 08903, USA

ABSTRACT

The behaviour of nesting and loafing herring gulls Larus argentatus was compared when the birds were exposed to supersonic transport, subsonic aircraft and normal colon), noises at Jamaica Bay National Recreational Area. No effects of subsonic aircraft on nesting gulls were noted. However, when supersonic transports flew over, significantly more nesting gulls flew from their nests, and they engaged in more fights when they landed compared with the other conditions. Man), eggs were broken during these fights, and subsequently eggs were eaten by intruders. At the end of the incubation period there were lower mean clutch sizes in dense sections (more potential for fights) of the colon)' compared with solitary nesting pairs of gulls. For loafing gulls, significantly more birds flushed when planes flew over compared with immediately before and after such plane noises.

INTRODUCTION

Birds are continually exposed to noises, although the sound levels recorded in nature are generally lower than those associated with urban cities. For humans, noise can be defined as any unwanted sound (Lee & Griffith, 1978), but for wildlife such a definition is useless. Instead, we measure sound pressure levels (in decibels) and assess potential damage by observing the behaviour of animals at different sound levels. Physiological damage, well known in laboratory experiments, acts on the auditory and central nervous system and later induces stress symptoms (Busnel, 1978). However, little but anecdotal data are available on free-living animals. Busnel (1978) states that some species (such as gulls on air fields) breed close to extremely loud man-made noises without ill effects, demonstrating that they 'prefer or do not avoid' noisy environments. Janssen (1978) categorises these animals as semi177 Environ. Pollut. Set. A. 0143-1471/81/0024-0177/$02-50 ~' Applied Science Publishers Ltd, England, 1981 Printed in Great Britain

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domesticated, although he does suggest that excessive noise levels might mask communication signals and secondarily interfere in breeding. Airports, with their associated jets, are particularly noisy. Since they are frequently placed in swampy land unsuitable for housing and industrial development, they are often in prime habitat for nesting birds, particularly gulls and terns. The frequency of noises and the number of decibels around airports are increasing with the growth in the number of passengers (and thus additional planes) averaging just under 20 ~o a year (Burns, 1973). Further, the addition of supersonic transport planes adds to the noise levels. Birds are assumed to habituate to the frequent loud noises of landing and departing airplanes, and only unusually loud noises are known to induce a reaction of alarm (Busnel, 1978). Dunnet (1977) examined the effects of helicopters and fixed wing aircraft on seabirds breeding on the Buchan Cliffs in the North Sea and found that incubating and brooding birds were not affected, although some attending, non-incubating and loafing kittiwakes Rissa tridactyla flew. A US Department of the Interior report (1969) on the environmental impact of the Big Cypress Swamp jetport, discussing jets flying at 170-1700 m over the park, reported that no birds were flushed and no disturbances observed, although few birds were in the area and no colonial species bred nearby. Noise levels ranged from 75 dB to 96 dB. Despite the close proximity of airports and breeding colonies, no quantitative data are available on the effects of the noise from the large subsonic aircraft common on large airports. In this paper I examine the effects of airplane noise on incubating and brooding herring gulls Larus argentatus nesting at Jamaica Bay National Recreational Area within 2 km of Kennedy International Airport. Large subsonic jets land every two or three minutes and the supersonic transport planes (SSTs) land daily. I was particularly interested in determining if the gulls responded differentially to these two types of plane. When the gulls returned to breed in March 1977, the Concorde (the current SST) was already landing daily, but it had not done so during the previous breeding season. In order to allow some time for habituation, I began observing when the gulls had been on the colony and exposed to the Concorde for over two months.

STUDY AREA AND METHODS

I observed herring gulls nesting on Canarsie Pol Island located in Jamaica Bay. The dredge spoil island is framed with Phragmites communis and trees. The higher sand and gravel sections are sparsely vegetated and contain a nesting colony of about 500 pairs of herring gulls and 12 pairs of great black-backed gulls L. marinus. Ample habitat exists and the gulls space out, preferring to nest close to the Phragmites along the edge of the island. The colony, located less than I km from one of the major runways of Kennedy Airport, is directly in line with the approach, although

RESPONSES OF HERRING GULLS TO AIRCRAFT NOISE

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this approach is not used continuously, due to wind conditions and administrative decisions. I began observing the behaviour of the gulls during late May 1977 when most (about 65~) gulls were in the last two weeks of incubation, and continued observations until the third week of the brooding phase. In one series of observations 120 pairs were observed nesting in a central area of the colony (100 x 100 m) bounded by Phragmites on all sides and separated from other sections of the colony. During 20 observation periods (2 h each) I recorded the noise level (in decibels on the A scale of a Scott Instruments Sound Level Meter), the number of birds flying over this area and the number of fights in 1-min sample periods at 20-rain intervals (provided no plane was overhead) and whenever any plane flew over. (No sonic booms were observed during this study.) When a plane flew over I also recorded the type of aircraft. The number of birds fighting ( = physical contact)was recorded for a group of 20 nesting pairs. These results were compared with other sections of the colony to ascertain that the results were representative. In a second series of observations (N = 20) in another section of the colony the number of fights were recorded which occurred in 1-min sample periods for a group of 20 nesting pairs under three conditions: when birds were undisturbed, immediately after an aircraft had flown over the island and immediately after a person had walked through the colony. A hundred nests were checked within the colony during early incubation and again in late incubation to determine mean clutch size. Herring gulls also loaf on many sections of the refuge. For 44 1-min samples before, during and after a plane went over, the following data were recorded: number of gulls, number feeding, number looking up, number wing-stretching and number taking off from the flock. This procedure was difficult, and I eliminated samples whenever all birds flew from the area, planes did not fly directly overhead, or planes flew over too close together temporally to obtain reasonable estimates of undisturbed behaviour. Non-SST planes included Boeing 707s, 727s and 747s. Although noise levels from these planes varied, no significant differences were observed among these planes and they were lumped for the purposes of this analysis. Noise levels for aircraft landing and taking off were pooled for this analysis. Normal colony noise refers to gull vocalisations plus low-level car noises from a highway 2 km away.

RESULTS

Nesting behaviour Normally herring gulls call as they fly over the colony, chase intruders and greet mates. These colony noises and the distant hum of traffic produce an average ambient noise level of 77dB(A) (Table 1). The noise levels recorded during the

JOANNA BURGER

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TABLE 1 BEHAVIOUROF NESTINGHERRINGGULLS WHEN EXPOSEDTO SUBSONICAIRPLANES,SUPERSONICTRANSPORTS AND NORMALCOLONY NOISES. NOISE LEVELSARE IN DECIBELSON THE A SCALE

Noise level (dB (A)) Mean b Range Birds flying overhead Mean Range Fights c Mean Range

Normal

Non-STT ° planes

SST

F

p

77.0 + 3.7 72-83

91-8 + 3'6 88-101

108.2 +__3.8 101-116

364.3

0.001

11.7 ___ 3"3 8-18

11.0 5- 2.9 8-16

137.0 + 40.5 36-220

173-6

0.001

0.1 + 0.4 0-1

0.2 + 0.3 0-2

6.3 + 1.6 2-7

121.3

0.001

* SST = Supersonic transport plane (Concorde). b Means + standard deviation. Each mean is based on 20 samples. c Fights within the first minute after the plane passed over, or during the 20-min sampling regime.

observations varied significantly: the SST levels were significantly louder than nonSST planes, which were louder than normal colony noises (Table 1). The Concorde produced sound levels of 116 dB(A) when it flew directly overhead and the ground vibrated noticeably. The number of birds flying over the colony is an indication of disturbance. Large numbers of gulls flying overhead often indicates the presence of a human intruder or a predator. In this study, the number of gulls flying over a prescribed area increased significantly whenever the SST flew over, but there were no differences between the non-SST planes and normal conditions (t = 0.31, Table 1). I then recorded the number of fights that occurred in 1-min samples following each observation (Table 1). Significantly more fights occurred following an SST disturbance compared with the other conditions. Although it can be argued that the increased level of aggression is due to more birds landing, the number of fights following an SST disturbance was 60 times greater than under normal conditions, whereas the number of birds in the air following an SST was only 12 times higher than under normal conditions. To confirm the increased levels of fighting following SST disturbances, I monitored the number of birds fighting in a group of 20 nesting pairs in another section of the colony. There were significantly more fights (F = 35.4, df = 2.23, p < 0-001) in the minute after the SST flew over ()? = 4.1 + 1.67) compared with normal conditions (.~ = 0.2 + 0-4) when no planes were overhead, and after a human had walked through the area (X = 1.0 + 0.90). In general the fights were longer in duration (1-2 min rather than 30 s) and eggs were broken (10 of 60 eggs) and eaten by other gulls. At the end of the first week of incubation the mean clutch size in the 100 sample nests was 2.76 + 0.32 while at the end of the incubation period the mean clutch size was 2.06 + 0.42 and only 60 ~ of these eggs hatched. Solitary pairs, separated by at

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181

least 50 m from other nests, had mean clutches of 2.80 + 0.34 at the end of the first week of incubation and 2.67 + 0.32 at the end of incubation (N = 15). These birds also flushed when the Concorde flew over, but they did not encounter other gulls when resettling after the disturbances.

Loafing birds throughout the refuge A flock of 50 to 150 herring gulls often loafed on the sand beach at the edge of the breeding colony at Canarsie Pol. These birds remained loafing whenever subsonic jets flew overhead, but they all flew whenever the SST flew directly overhead. Samples on loafing birds were taken at a variety of locations around the refuge. For 1-min samples, pre-plane noise levels varied from 42 to 85dB(A) (.~ = 60.8 + 10.8), plane sound levels varied from 80 to 116 (.~" = 97 ___ 12.3, including the SST) and post-plane noises (1 min after the plane went over) varied from 52 to 90 (.~ = 61.0 + 10.3). Significant differences existed, with plane noises being louder than sound levels before and after (F = 30-3, df = 2-42, p < 0.001). There were no differences in the number of birds which fed, looked up, or wing-stretched (Fisher Exact Test). With respect to the number of gulls that flew, more gulls flew during the plane noise than either before or after (Fisher Exact Test, p < 0.05). Birds that flew immediately circled and landed in the loafing flock; thus they do not represent birds leaving for foraging areas or the colony. In an average sample, no more than 1 ~ of the gulls flew in the minute before or after the disturbance, while during the plane noise up to 20 ~o of the gulls flew. Thus, even in gulls away from the breeding colony, loud plane noises resulted in more gulls flying up from the flock. DISCUSSION

Sound levels In this paper I distinguish between two kinds of aircraft (the SST and subsonic jets) which elicited different responses in nesting herring gulls. Subsonic jets produce two kinds of noise: one resulting from the turbulence generated by the interaction of the high velocity jet with the stagnant atmosphere and a high intensity whine caused by high-speed rotation of the engine's multibladed fan compressor (White, 1975). Subsonic planes generate 85 to 100 dB(A) on approach and 94 to 105 dB(A) on takeoff (measured at 300m, White, 1975). The supersonic transport planes produce sonic boom in addition to subsonic noises. The SST sound characteristics are substantially different from those of other planes, with greater low-frequency acoustic energy and higher take-off noise levels capable of inducing higher levels of structural vibrations (Wiggins, 1974; Wesler, 1975, 1978). Since many authors report significant noise-induced structural vibrations (see Wesler, 1975), the potential for vibration of eggs and birds exists. Thus, the dangers from supersonic transport planes derive from their loud noises, vibration capabilities and sonic booms.

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In this study I made observations on herring gulls only when the planes passed directly over the nesting island. The sound levels recorded for subsonic planes are within the range recorded by White (1975, see above) and by Burns (1973) at London's Heathrow Airport. The supersonic transport is a noisier plane, resulting in noise levels of up to 116 dB(A) when directly overhead.

Behavioural effects on herring gulls Information on the effects of noise on birds is notably lacking in the literature, although Grue (1977) stated that birds can sometimes be affected and Lee & Griffith (1978) reported that few birds resided along power lines that hummed. Several authors (see Busnel, 1978) noted that gulls are particularly non-responsive to noise, frequently reside near airfields and seem to pay no attention to the Concorde. These reports are generally anecdotal and non-quantitative. For most gulls sitting around airports it is difficult to assess their responses accurately for several reasons: (1) gulls often fly up for no apparent reason and without quantitative data it is impossible to determine if flight activity increases with increasing sound levels; (2) gulls often behave as a flock, and if one flies they all fly, making it difficult to correlate sound levels with differences in the percent of the flock flying and (3) gulls in a flock are loafing and not engaged in reproductive activities which might be more sensitive to noise. Information on the effects of noise on nesting birds is particularly limited, although Dunnet (1977) studied a seabird colony in Scotland exposed to low-flying planes. The exploration of the North Sea oilfields increased air traffic, particularly helicopters. He counted the number of birds present on the nesting stacks immediately before and after the passage of aircraft, although he did not record sound levels. He found no evidence that aircraft flying at heights of about 100m affected the presence of incubating and brooding birds, although the attending (but non-incubating) adult kittiwakes sometimes departed with these disturbances. Nearby loafing and roosting kittiwakes also took to the air in response to planes, but their responses were not compared with and without plane noises. Kittiwakes also often took offwhen no planes flew over. His data primarily refer to helicopters which I would not expect to be as disturbing. First, helicopters are not as loud (dB(A) averages of 75, M. Gochfeld, pers. comm.) and, secondly, they do not produce the low-frequency noise levels characteristic of supersonic transports. Helicopters flying directly over nesting colonies of herons, egrets and ibises are known to flush incubating birds although they frequently return within 5 rain (see Kushlan, 1979). In the case of the response of kittiwakes to helicopters, the birds had frequent exposure to the helicopters, whereas the herons examined were not exposed to frequent helicopter disturbance. In the breeding colony studied, the gulls' behaviour when subsonic planes passed was similar to normal colony behaviour. However, when supersonic transports passed directly over the colony significantly more birds flew up and gulls engaged in

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more fights than under normal conditions. Similarly, there were more fights in the minute after the SST passed than when no plane passed and after human disturbance. Thus nesting herring Bulls responded differently to the SST compared with all other planes. These responses may have been due to the noise levels, frequency levels of the sounds, or to vibrations. The level of response reported here was similar to that observed in one day in April at the start of egg-laying (unpublished data) suggesting that the gulls did not habituate during the first reproductive season following exposure to the SST. Since the SST only lands once or twice daily, and lands on the nearest runway only some of the time, the frequency of exposure might not be sufficient for habituation. This aspect requires further investigation. Continued exposure in other nesting seasons may result in habituation, although the low-frequency, loud noises of the SST may preclude such habituation. In a study of humans at Heathrow Airport in London, Burns (1973) found that over half of the people who had been living near the airport for several years registered the highest possible annoyance scores when exposed to noise levels of 103 to 108dB(A), the usual levels of the SST. For birds nesting within the colony the mean clutch size decreased markedly during incubation while pairs which were isolated did not exhibit such a decline. The decrease noted for the gulls nesting together (X of 0.7 per nest) is greater than normally expected during the incubation period (about 0.2 per nest, unpublished data; Burger & Shisler, 1979). I propose that the lower clutch size--and thus the lowered reproductive success--resulted from the frequent fights which followed SST disturbances rather than directly from noise or vibration levels. Eggs are known to be resistant to sonic booms (see Wilson, 1971 ; Cottereau, 1978; Lynch & Speake, 1978). In this study I often observed eggs being broken when returning adults engaged in prolonged fights. Normally such fights do not occur, because adults return to their nests at different times, but the SST synchronises these landings and close nesting pairs may land in their neighbour's territory. Even loafing gulls responded differentially to plane noise. For flocks in which all gulls did not flush, more gulls flew during the plane noise than immediately before or after. However, this statistical response would not have been obvious from casual observations, which may account for the commonly held belief that gulls are not affected by jet noises (see Busnel, 1978). The results of this study suggest that we need to develop behavioural measures of the effects of noise on both loafing and nesting birds and to compare reproductive success under controlled conditions, before concluding that birds are unaffected by noise.

ACKNOWLEDGEMENTS I am especially grateful to M. Gochfeld for help in all phases of the study. I thank S. Brady and W. Wander for field assistance. I thank G. M. Dunnet and W. E.

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S o u t h e r n f o r c o m m e n t s o n a n e a r l i e r v e r s i o n o f this m a n u s c r i p t . T h i s p r o j e c t w a s p a r t i a l l y s u p p o r t e d b y a c o n t r a c t ( C X 1600-8-0007) f r o m t h e N o r t h A t l a n t i c R e g i o n a l Office o f the N a t i o n a l P a r k S e r v i c e a n d I t h a n k P. A . B u c k l e y for his c o n t i n u e d i n t e r e s t in t h e p r o j e c t .

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