Bird Behaviour
Bird Behaviour
Description
Flight crew and air traffic controllers don’t need to be amateur ornithologists to gain some useful awareness of bird behaviour! This summary aims to highlight some of the potentially useful generalisations that experts have deduced from their years of dedicated studies.
Geese in flight over Northern Territory, Australia, April 2014. Source: Wikicommons. Author: GDW.45
Birds as Objects
Bird size is not always a good indication of their potential to cause impact damage. Body density is what most determines damage potential and there are some quite large variations in the density of some species. The Laughing Gulls, which used to frequent New York JFK until control was improved many years ago, have a higher density than the Herring Gulls, which are frequently present around other airports, so that, even though they are only one third of the size of Herring Gulls, they were found to cause similar damage. Starlings are another common bird species which have an even higher density than laughing gulls and have been found to cause correspondingly greater damage than other similar-sized, and some larger, bird species.
Bird Awareness of and Response to Aircraft
Despite their much smaller size, the vision and hearing of most birds is broadly similar to humans, with the exception that the high frequency cut off of their hearing is lower. Comparable discrimination and range of vision to humans is achieved by their eyes being around 15% of head weight compared to the 1% in humans, whilst having similar head/body weight ratios. A bird's sense of smell is less developed though, which can perhaps be associated with the unpalatable smells which are associated with some of their food sources.
The effect of this is that birds are aware of aircraft. However, since aircraft are not predators, birds are not automatically cautious in their proximity unless previous experience (direct observation) of them as a hazard has produced such a reaction. In addition, birds which regularly encounter aircraft without such experience, become habituated to them and even less cautious than they might otherwise be. The most likely birds to feature in actual impacts include young birds where there are breeding colonies in the vicinity of an airport.
The most important feature of the execution of last-minute avoidance manoeuvres by birds has been shown to be their unpredictability. Avoidance of aircraft in straight and level flight seems to present little difficulty but at the low altitudes at which birds are most likely to be encountered, aircraft are usually climbing or descending. This is where the unpredictability comes in. Observational evidence suggests that the tendency to attempt to dive or free fall beneath an aircraft rather than climb above it, which is more marked for some species than others, is not reliable enough to support a corresponding avoidance-climb response. Such a response might still lead to a strike, whilst the aircraft would be either at an increased rate of climb or in the transition from descent to climb, and aircraft performance margins would be reduced with a greater risk of significant consequences arising from strikes. Part of the underlying explanation for unpredictable aircraft avoidance is that birds do not always seem able to perceive aircraft as being in motion. Exceptionally, some hawks and eagles have been seen to ‘attack’ an aircraft which they encounter.
Bird Flight Habits
Small birds ‘going somewhere’ tend to fly at around 20 kts whereas larger birds, such as geese, may reach speeds of up to 40 kts. Day to day flight altitudes for most birds are in the range 30 feet to 300 feet above ground level (agl). Outside that range, they rarely exceed 1000 feet agl unless on long distance migration flights. These typically occur at a 5000 - 7000 feet altitude, subject to terrain, but have sometimes been detected at over 20000 feet.
When and Why Birds Fly
Essentially, day-to-day bird flight is about food. For most this means insects and other invertebrates either from the ground and foliage or in flight. Vegetation is the next most popular food source. Others species depend upon small mammals and amphibians or on fish, carrion or rubbish dumps.
Birds fly mostly by day since relatively few species are adapted for night feeding. It is generally estimated that around 90% of recorded bird strikes occur during daylight, but since this is also when most aircraft fly, strike rates can sometimes actually be higher at night. Routine daytime activity, being feeding-related, is at its greatest until late morning. The hazard of flocking may occur in association with attractive localised feeding areas which, as in the case of agricultural activity, can sometimes be quite transient and effectively unpredictable. Once the usual morning food intake is over, birds of many species tend to indulge in ‘loafing’ or idling in or around large, open, flat and mainly undeveloped areas of which airports are a prime example. Airports often have the additional attraction of marginally uneven surfaces which can provide ephemeral shallow water expanses which make ideal drinking and bathing pools for birds. Near dawn and dusk, there may be specifically identifiable transit routes from and to communal roosts for some species.
‘Poor’ weather conditions tend to reduce bird feeding activity and the transit ‘traffic' associated with it. The majority of birds (although not all the species which represent the most significant aircraft strike hazard) are insect feeders. This means that the effect of heavy rain is particularly noticeable, since many insect species will ‘take cover’ in foliage or surface vegetation. However, and especially if this rain is in the form of limited duration showers, the aftermath of a shower often results in mass insect re-emergence and the opportunity for birds to indulge in a ‘feeding frenzy’!
Specific bird congregations may also be associated with all-day feeding at rubbish dumps and when birds use local convection sites to soar with little energy expenditure either to ‘loaf’ or, in the case of hawks and vultures, to loiter for food. Since large airports can be significant ‘heat islands’ compared to the surrounding local environment, they are very conducive to a local convective effect even without the visual evidence provided by cumulus-type cloud formation being initially evident.
Migration - mainly away from the Equator in spring and towards it in autumn - is another matter and much of this movement takes place at night as well as by day. As well as occurring as a group activity, it often involves generalised routes or flyways. The occurrence of migration, in terms of timing and detailed routing, are tied to weather conditions; this is because migratory movement during extremes of weather - abnormally cold temperatures, rain, severe wind turbulence, or thick fog - are not conducive either to bird energy budgets or bird navigation. The occurrence of significant tailwind components at migration altitudes can affect the numbers of birds en route very significantly. Finally, migration periods can also be associated with large congregations of birds over mountain ranges and coastlines where they may await an optimum transit opportunity and often be able to do so with the benefit of significant local thermals. State Aeronautical Information Publications (AIPs) often include information on known regular migration routes, ‘holding areas’, and the timing of migration.
Bird Species Specifics
Serious hazards, where control of a larger aircraft might be lost due to bird impact, are likely to arise if the birds encountered are large and/or have high density and have a tendency to flock. Geese and swans are the primary species groups with these characteristics. The hazard for transport aircraft where birds are neither large nor of high body density and are not flocking is usually slight.
Of significant species, gulls are the most generalist feeders and often live in colonies of thousands. They are supremely adaptive to change. As a group, they are one of the main reasons why coastal airports usually have much higher strike rates than inland airports. Since they are of significant weight - in the range 0.7 to 2.25 kg - they feature prominently in the damaging strike rates for many airports.
Both doves and pigeons are increasingly found around many urban airports and, at up to 0.4kg in weight, are capable of causing damage to smaller aircraft and engines even in individual strikes.
Waterfowl (including geese, swans and ducks) range from 2.25kg to 9.5kg in weight and are inclined to flocking behaviour which combines with their weight to produce one of the most potent multiple strike hazards to aircraft. Geese in particular often cause major aircraft engine damage.
Raptors (including vultures, hawks, eagles and falcons), some of which may exceed 5 kg in weight, are also capable of causing major impact damage, even as individual birds. They may also be encountered at higher altitudes than most other species except migrating waterfowl.
Accidents and Incidents
The following events in the SKYbrary database may be of interest:
Migration
On 8 January 2003, a DHC8-400 sustained multiple bird strikes during a night visual circuit at the Medford airport, OR, USA, resulting in loss of flight displays, multiple false system warnings and the shattering of the LH windscreen. The Captain sustained significant facial injuries and temporary incapacitation with a successful approach and landing being completed by the co-pilot.
Flocking Birds
On 31 July 2012, a Boeing 737-900 struck a single large bird whilst descending to land at Denver in day VMC and passing approximately 6000 feet aal, sustaining damage to the radome, one pitot head and the vertical stabiliser. The flight crew declared an emergency and continued the approach with ATC assistance to an uneventful landing. The bird involved was subsequently identified as a White Faced Ibis, a species which normally has a weight around 500 gm but can exceptionally reach a weight of 700 gm. The hole made in the radome was 60 cm x 30 cm.
On 19 January 2013, a Rolls Royce Trent 700-powered Virgin Atlantic Airbus A330-300 hit some medium sized birds shortly after take off from Orlando, sustaining airframe impact damage and ingesting one bird into each engine. Damage was subsequently found to both engines although only one indicated sufficient malfunction - a complete loss of oil pressure - for an in-flight shutdown to be required. After declaration of a MAYDAY, the return to land overweight was completed uneventfully. The investigation identified an issue with the response of the oil pressure detection and display system to high engine vibration events and recommended modification.
On 3 August 2006, a Qantas Boeing 767-300 encountered a large flock of birds during rotation and sustained multiple strikes on many parts of the aircraft. Left engine vibration immediately increased but as reducing thrust also reduced the vibration, it was decided following consultation with maintenance to continue to the planned destination, Sydney.
On 29 July 2017, an Antonov AN-74 crew sighted several previously unseen large eagles rising from the long grass next to the runway as they accelerated for takeoff at Sao Tome and, concerned about the risk of ingestion, made a high speed rejected takeoff but were unable to stop on the runway and entered a deep ravine just beyond it which destroyed the aircraft. The Investigation found that the reject had been unnecessarily delayed until above V1, that the crew forgot to deploy the spoilers which would have significantly increased the stopping distance and that relevant crew training was inadequate.
On 10 November 2008, a Boeing 737-800 about to land at Rome Ciampino Airport flew through a large and dense flock of starlings, which appeared from below the aircraft. After the crew had made an unsuccessful attempt to go around, they lost control due to malfunction of both engines when full thrust was applied and a very hard impact half way along the runway caused substantial damage to the aircraft. The Investigation concluded that the Captain s decision to attempt a go around after the encounter was inappropriate and that bird risk management measures at the airport had been inadequate.
Further Reading
Transport Canada
- TP 13549 2nd edition Chapter 3: Sharing the Skies
UK CAA
- UK CAA Paper: Large Flocking Birds
Others
- EGAST Safety Material on bird strike risks for GA operations: Bird strike, a European risk with local specificities, Edition 1 - Germany, 10 July 2013.
- Bird strikes and aircraft fuselage color: a correlational study, E. Fernandez-Juricic, J. Gaffney, B. Blackwell, P. Baumhardt, 2011.
- Strategies for prevention of bird strike events, R. Nicholson, W. Reed, Boeing AERO magazine, Q3 2011.
- Airport Practice Note 6 'Managing bird strike risk', by the Australian Airport Association, September 2015
- ICAO Electronic Bulletin: 2008 - 2015 Wildlife Strike Analyses, 2017
- "Red Light, Green Light: Study says birds' vision may be less sensitive to red navigation lights," by Linda Werfelman, FSF AeroSafety World, 12 Dec. 2017.
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