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Introduction and General Information

While the finding of oiled birds may be an initial indication of the occurrence of some spills (D133.1.w1) (and may be the only indication of a small spill), assessment and monitoring of an oil spill incident should usually be the first activity in oiled wildlife response. (D183.w6)

"A competent incident assessment will be key to a successful response, and will help determine the magnitude and nature of the response needed." (D183.w6)

Priority oiled wildlife response actions in the event of an oil spill will be decided on the basis of the assessment of the incident, both initially and on a daily basis as the incident develops and progresses. (D183.w6)

In the event of a spill it is important to determine:
  • Whether wild animals (birds, mammals, reptiles, amphibians) are at risk from the spill;
  • The number of individuals oiled or at risk of oiling;
  • Any risk to endangered, rare or culturally important species or populations;
  • Appropriate wildlife-orientated response activities;
  • Priorities for response;
  • Whether or not different response options are appropriate (e.g. whether uncontaminated habitat is available nearby if birds are to be hazed from oiled areas);
  • The level of response required (see Tiered Response below).

(D10, D159.III.w3, D160.3.w3, D183.w6)

It is suggested in D183.w6 -(A Guide to Oiled Wildlife Response Planning - IPIECA Report Series Volume 13 - full text provided) that incident appraisal should involve the collection of the following key information: 

  • size of spill, type of oil, expected drift and weathering of oil released, etc.; 
  • location of oil in relation to sensitive wildlife areas;
  • time of year/season (affects behaviour of animals and habitats they may be using);
  • potential threat to wildlife at sea and on the shore;
  • potential for further pollution;
  • wildlife and habitat already impacted;
  • estimate of the number of losses at sea (unreported casualties);
  • resources available;
  • need for external support;
  • consideration of the need for triage/euthanasia.

(D183.w6 - Full text provided)

Published Guidelines linked in Wildpro

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Tiered Response

General oil spill response uses the "tiered response" concept, in which spills are classified as Tier One (small spills requiring only local resources for clean up), Tier Two (medium sized spills requiring a regional response) and Tier Three (large spills in which a national response is required). (W468.Jan2003.w1)

The requirements for oiled wildlife response can also be considered using the tiered response concept. 

  • When individual wild animals are oiled, or when a spill contaminates only a few individuals, local wildlife rehabilitators/wildlife hospitals may be able to respond without any assistance. (D214)
  • When more animals are involved, beyond the capabilities of local organisations, a regional response may be required. 
  • For spills involving hundreds or thousands of wildlife casualties, a national or even international response effort may be needed.

It has been suggested that, while the initial response assessment should indicate the required scale of response, if there is doubt regarding the level of response then a "worst case" scenario should be prepared for, since it is easier to reduce the scale of the operation than to increase it. (D183.w7)

Associated techniques linked from Wildpro

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Contingency Plan Data

Data should be available in contingency plans indicating:
  • Locations of species at risk of oiling;
  • Presence at different seasons, including changes during the spring and autumn migratory periods;
  • Data on abundance (i.e. actual numbers counted per calendar week) and when the last survey was conducted;
  • Data on which species and species groups (e.g. gulls, waders) are present, so that areas where the species most vulnerable to oil spills are found, and where colony breeding birds nest, have been identified;
  • Data on endangered and rare species, to allow rapid protection of these species.


Further information on data which should be included in contingency plans is included in the description of the Data Section within Oil Spill Contingency Planning - Sections of the Wildlife Response Contingency Plan

Associated techniques linked from Wildpro

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Oil Movements and Resources at Risk

The threat to wild animal populations will vary depending on exactly where the oil is spilled and its rate and direction of movement. For appropriate assessment it is therefore necessary that those responsible for wildlife response assessment have access to accurate, timely information regarding the position and actual and projected movement of any oil slick. 
  • This requires good communication and liaison with those responsible for the general oil spill response, who should have this information. Integration of the oiled wildlife response plan with the general oil spill response promotes accessibility of such information. (D183.w6)
  • Information about oil movement is also required in order to avoid potentially counter-productive response activities, such as dispersing birds into areas which might become contaminated. (D160.3.w3)
Associated techniques linked from Wildpro --

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Species and Numbers Threatened or Affected

When a spill is notified, the available data bank should be consulted. If there are notable populations of water birds or vulnerable mammals in the area, or if there is insufficient data available, then preliminary surveys of the area are required. (D10)

Both aerial and ground reconnaissance surveys should be conducted to determine wildlife resources at risk. The main objectives of the surveys are to evaluate the number, species and locations of wildlife which may be affected by the spill, and the extent of oiling of individuals which has already occurred. (D60.6.w6, D160.3.w3)

Initial surveys should be conducted as soon as possible, preferably within three hours for aerial surveillance and within six hours for ground surveillance. (D10)

Aerial surveillance

For aerial surveillance, flights at under 100m will allow accurate identification of bird species. Aircraft involved in wildlife surveillance should fly at about 100 km/hr. Surveillance by helicopter is ideal, but it can also be carried out from fixed-wing aircraft. An experienced ornithologist is required [and should be identified prior to the spill] to carry out the survey effectively, in order to identify species and record relevant characteristics, including behaviour. This survey will not allow assessment of the number of oiled birds, but may indicate locations of birds in difficulties. (D10, D160.3.w3)

The survey should provide information on: (D10)

  • The number of birds present in the area (hundreds, thousands or tens of thousands). this information may make a huge difference in personnel and equipment required.
  • Species present, particularly endangered species and those which are very vulnerable to oiling. Presence of large numbers of vulnerable species suggests that preparations should be made for opening of cleaning and rehabilitation centres.
  • The location of birds in relation to the distribution and movements of the oil. If oil will drift towards large concentrations of birds within hours, then rapid initiation of deterrence is required.


Ground surveys

Preliminary ground surveys have as their primary objectives: (D10)

  • Evaluation of the percentage of birds already contaminated;
  • Detection of birds in difficulties;

Specific information should be gathered on: (D10)

  • The number of birds of each species in the critical contaminated area
  • The exact location of birds;
  • Abnormal behaviour of birds, such as intensive preening, or inability to fly when approached;
  • Whether birds are contaminated with oil, and if they are then the degree of contamination.


Associated techniques linked from Wildpro

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Response Options

Response options when an oil spill occurs in which wild animals may be oiled include keeping oil and wild animals apart from one another, and/or catching and treating oiled individuals.

Oil and wild animals may be kept apart from one another by:

  • Containing the oil, e.g. using booms; (D188, D192)
  • Removing the oil from the environment; (B20.13.w10)
  • Removing the animals from the area in which oil has been spilled or into which it is moving, by deterrence or preemptive capture. (D183.w6)

These options are discussed further in Preventing Oiling of Wildlife

Individual oiled animals may be caught and taken for cleaning and rehabilitation. 


  • For maximum effectiveness of response options, they must be instigated in a timely manner. (D10, D160.3.w3, B188, D9, D160.4.w4, J313.40.w3, P62.1.w1)
  • In the event of a spill, decisions on response options will have to be made based not only on pre-recognised priority areas but also on practicalities: whether or not it is possible to protect an area, and allowing for if priority areas have already been impacted. (D60.2.w2)
  • Decisions on response options may have to balance the needs of different types of organisms: sometimes what is best in order to reduce oiling of birds may conflict with what is best for other fauna and flora affected by the oil. (D193)
Associated techniques linked from Wildpro

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Estimating Mortality due to an Oil Spill

"It is generally accepted that there is no clear relationship between the volume of oil spilled at sea and the resultant number of oiled seabirds." (D159.II.w2)

It is not easy to estimate mortality due to an oil spill. (D159.II.w2)

  • Not all oiled birds are washed ashore; many carcasses of birds which die at sea never reach the shore. (D183.w6)
  • Many birds may be oiled by a relatively small spill or conversely only a few birds may be affected by a large spill. (D159.II.w2)
  • While extensive work has been carried out on models to estimate total mortality following a spill, those developed to date are specific to a particular site. (D159.II.w2)
  • There is no "multiplier" which can be used in all spills to estimate total mortality from the number of carcasses detected. (D159.II.w2)

Limitations of beach surveys

Reasons why many carcasses are not found on shore include:

  • Loss of birds at sea with carcasses never reaching the shore. (D159.II.w2, D183.w6)
  • Rewash - individuals washing ashore then refloating on a later tide. (D159.II.w2, D183.w6)
  • Predation and scavenging of carcasses. (D159.II.w2, D183.w6)
    • Carcasses may be scavenged in under 24 hours. (D159.II.w2)
    • Losses due to scavenging of small birds will tend to be greater than losses due to scavenging of large birds, since the whole body may be carried off, rather than pieces eaten with parts remaining to be found by search and collection personnel. (D159.II.w2)
    • In general there are more scavengers on coastal beaches than in estuaries, so more carcasses will be scavenged from coastal beaches. (D159.II.w2)
  • Search efficiency: this will be affected by the search method (e.g. on foot or by vehicle), by the size and colour of the casualties (small individuals may be less easy to spot than larger ones) and because individuals may seek shelter in dunes or vegetation some distance from water and away from the main search area. (D183.w6)
  • Oiled individuals may leave the area after oiling and never be found; (D183.w6)
  • Members of the public may remove or bury carcasses. (D159.II.w2, D183.w6)

Assessment of birds lost at sea; factors affecting recovery rates

  • In near-shore incidents most birds are likely to still be alive when arriving on shore and losses at sea may be considered insignificant. however for offshore spills such losses may be considerable. (D183.w6)
    • It may be necessary to make an estimate of the number of oiled casualties lost at sea. (D183.w6)
    • This may involve drift experiments to assess the likelihood of oiled birds being washed ashore. (D183.w6)
  • The species involved in a spill may affect recovery rates. Pelagic species tend to avoid coming out onto beaches even when oiled, therefore are more likely to die at sea than are bay or estuary species which are more likely to haul out when oiled. (D159.II.w2)
  • Wind direction affects the rate of recovery: onshore winds will tend to push animals ashore, while offshore winds may keep them away from shore for longer. (D159.II.w2)
    • The longer that a carcass is kept away from shore, the more likely it is that it will sink before becoming beached. (D159.II.w2)
  • The greater the distance that a carcass must drift to reach the shore, the greater the chance that it will sink before becoming beached: in general, even in calm water carcasses will begin sinking after 10 to 14 days and sinking will occur sooner in rough water. (D159.II.w2)
Associated techniques linked from Wildpro --

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Authors & Referees

Authors Debra Bourne MA VetMB PhD MRCVS (V.w5)
Referee Dr Virginia Pierce (V.w73)

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