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Introduction and General Information

Post-release monitoring is an important and integral part of oiled wildlife rescue and rehabilitation. (B363.12.w12, D160.7.w7)
  • Data produced by such follow-up assist in the development of release criteria. (D133.7.w7, D160.7.w7)
  • Data from post-release monitoring allow evaluation of the effectiveness of rehabilitation. (B363.12.w12, D60.7.w7, D159.III.w3, D160.7.w7, D208.7.w7 P14.5.w8, P24.335.w12, P24.335.w21)
  • Data from post-release monitoring also assist in understanding the long-term effects of oil on wildlife (e.g. on survival and breeding success). (B363.12.w12, D133.7.w7, D160.7.w7, P14.2.w5, P24.335.w12)

If possible, in addition to basic information on survival, data should be collected on behaviour, including movement, reproductive success, and other data as appropriate. (B363.12.w12, D208.7.w7, P24.335.w21)

N.B. "any research done should not: significantly extend the length of time that an animal remains in captivity, add unnecessary pain or stress to the birds while under care, or needlessly risk the animal's chance for survival following release." (D185.w7)

Published Guidelines linked in Wildpro

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Ringing/Banding/Tagging Return Data

Leg rings in birds and tags in mammals have been used for many years for identifying individuals; implanted microchips (passive transponders) have been used more recently.
  • Banding is an inexpensive form of post-release monitoring. (P24.335.w21)
  • The utility of rings and tags is limited because:
    • The banded or tagged individual has to be found and the identifying band or tag seen clearly to allow identification; (B379.38.w38, P24.335.w21)
    • Rings or tags can be lost. (J319.26.w1, B379.38.w38)
    • Ring return data does not give information on whether or not a bird has bred successfully. (P62.14.w1)
    • Ring return data may or may not provide information about whether a bird has migrated successfully. (P62.14.w1)
  • Microchips have the advantage of a unique identification number, but they require a special instrument for reading the microchip, chips from different manufacturers do not always pick up different chips, and they will not be found unless someone has the animal in the hand and specifically checks for the microchip with the correct reader. (V.w5)

Before release, all oiled and rehabilitated birds should be fitted with an appropriate ring for permanent identification. This allows the rehabilitated bird to be identified if it is found, dead or alive, at a later date. (D60.7.w7, D133.7.w7,  B363.12.w12, P24.335.w21)

  • Bands are "returned", i.e. found on a dead bird, by hunters, by surveys such as beach walks, and by individuals. Occasionally band data is available for a live bird seen in the wild. (D133.7.w7)
  • Liaison with national organisations which are responsible for co-ordinating animal identification and post-release monitoring schemes is advisable wherever possible.
    • In the UK, the British Trust for Ornithology runs the national bird ringing scheme; birds must be ringed by licensed ringers. (D7)

To maximise the usefulness of band return data, details of the permanent identifying ring should be coordinated with the complete capture, care and pre-release assessment record for the individual animal.

  • Note: Banding is a relatively simple method of post-release monitoring, however, due to low band return rates, data gathered cannot easily be used to determine survival. (B197.15.w15, D133.7.w7, D160.7.w7)
    • Banding returns (finding of dead, ringed (banded) birds, is generally very low. On its own, banding return data may not be sufficient to allow evaluation of oiled wildlife response efforts. (B363.12.w12)
    • Typically less than 1% of bands are "returned". (D133.7.w7)
    • It is probable that those individuals which die following release often die quite soon and close to the release point, and are more likely to be found and included in the data, while those which survive long term are more likely to disperse widely from the release site. Therefore ring recoveries are likely to be biased towards the non-survivors, so data based on this is probably unrepresentative of the true survival rate. (P14.5.w8)
    • Bands can be useful for developing long term histories or where there is a high probability of recoveries/sightings. (P24.335.w21)
    • Returns from game (hunted) species may be more likely than returns from coastal or pelagic species which are generally inaccessible. (P62.14.w1)


  • Numbered tags can be used in mammals in a similar way to bands in birds, although there are fewer organised large scale studies for mammals.
  • Pinnipeds may be marked using flipper tags. (P14.2.w5, P14.3.w15)
    • In the UK, suitable tags for seals are provided by the Sea Mammal Research Unit; tags are marked with an identifying number on one side and "Inform London Zoo" on the other. (V.w21)
  • Pinnipeds may also be marked using head tags (seal "hats"), also designed by the Sea Mammal Research Unit, which are brightly coloured and embossed with a two-figure number, thus easily readable at up to 50 m. (B379.38.w38)

Benefits of tagging pinnipeds have been listed as: (P14.2.w5, P14.3.w15)

  • Monitoring the survival rate of released oiled individuals;
  • Possibility of re-sighting allowing visual assessment of released individuals;
  • Possibility of necropsy if an animal is located soon after death, allowing determination of the cause of death [and correlation of necropsy findings with antemortem records] and possible further information regarding the effects of oil exposure;
  • Tracking of migration and of patterns of foraging and diving;
  • Possibility of monitoring future reproductive performance.

(P14.2.w5, P14.3.w15)

Associated techniques linked from Wildpro

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Observation using Coloured Bands or Dyes

Coloured leg bands (or in mammals, coloured ear tags or other marks), are used in some studies to allow identification of individual animals at some distance, particularly using binoculars or a telescope. Sometimes large numbered bands are used (e.g. Darvic bands on waterfowl). In other studies, combinations of plain coloured bands are used on each bird to identify individuals.
  • Coloured bands may allow behavioural observation of birds after release. (B363.12.w12, D133.7.w7, D160.7.w7)
  • This method can only be used to monitor birds which remain in the observation area (i.e. remain close to land, and do not migrate), and requires a large amount of observer time. (D133.7.w7, D160.7.w7)
  • Colour bands can be useful with larids and shorebirds. (P24.335.w21)
  • Coloured banding is an inexpensive form of post-release monitoring. (P24.335.w21)
  • N.B. a permit/licence may be required for use of coloured bands. (P24.335.w21)

Note: Coloured bands must be used with caution, since band colour has been shown to affect mate choice and/or survival in some species of birds. (B105.15.w2)

Coloured dyes

  • Coloured, persistent organic dyes can also be used, sprayed onto feathers. (P24.335.w21)
  • These are suitable for use in species which are expected to remain close to where they were found and when there is a reasonable expectation that birds will be seen after release (P24.335.w21)
  • Dyes fade with time, or are lost with feathers during moulting. (P24.335.w21)
  • Dyed feathers may be more visible than coloured rings. (P24.335.w21)
  • Care must be taken in dye choice, since some colours may encourage predation or intraspecific aggression. (P24.335.w21)
Associated techniques linked from Wildpro

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Radiotelemetry, Satellite Tracking and GPS

"Wildlife radio-telemetry may be defined as the transmission of information from a transmitter on a free-ranging wild animal to a receiver." (B380.1.w1)

Radiotelemetry, satellite tracking and GPS systems may be used to follow movements of birds, mammals, reptiles and even amphibians. (B379.38.w38, B380.7.w7, P14.2.w5, J313.32.w1, D217)

A transmitter is attached to, or implanted in, an animal before it is released. Once activated, the transmitter emits a specific signal which can be tracked using an aerial or by satellite (depending on the transmitter used). (D133.7.w7, D160.7.w7, B379.38.w38, B380.2.w2, B380.5.w5, D217)

  • A transmitter may be attached to the animal using adhesive, sutures or a harness, or combinations of these (e.g. sutures plus adhesive), or may be surgically implanted. (B23.3.w7, B379.38.w38, B380.3.w3, J48.62.w1, D133.7.w7, D217)
    • Transmitters attached to fur or feathers will be lost after moulting, which is a disadvantage for long-term studies. (B379.38.w38, D217)

The main advantages of radiotelemetry and satellite tracking are:

  • The individual can be tracked over a prolonged period, sometimes months to years (depending on battery life etc.). (D133.7.w7, D160.7.w7)
  • It is not necessary to make visual contact with the animal in order to track it.
  • Depending on equipment used, it may be possible to gather additional data about the animal, not just location but e.g. depth and duration of dives, body temperature etc..
  • Animals can be tracked over very long distances where satellite transmitters are used.
  • Very accurate tracking can be achieved with radio-telemetry,

(B23.3.w7, B379.38.w38, B380.2.w2, B380.5.w5, D217)

The main disadvantages of radiotelemetry and satellite tracking are:

  • Equipment required, which is specialised and (variably) costly, particularly for satellite tracking.
  • Possible negative effects on the animal from:
    • The weight of the transmitter;
      • Complete radiotracking tags, including battery and antenna, are now available down to weights as low as 0.35 g. [2004 data](W492.Feb04.w1)
    • The attachment method;
    • "Drag" of the transmitter and/or harness (in aquatic animals).
  • Limited effective time (based on length of battery life).
    • Battery life available for lightweight tags is longer than it used to be. (W492.Feb04.w1)
    • Extended life is possible using intermittent transmission (limited time in each day, or e.g. transmitting only one in three or five days), or by (for appropriate species) using solar cells. (B23.3.w7, D217)
  • The time required for tracking using antennae (including the costs associated with this).
  • Training required for successful use of tracking antennae.
  • Limited distances over which radiotelemetry signals can be detected.
    • For remote, pelagic and migratory species, satellite transmitters are preferable. (B379.38.w38)
  • Limited accuracy of satellite transmitters, making them unsuitable for studies on e.g. precise habitat use.

(B379.38.w38, B380.2.w2, B380.5.w5, B380.7.w7, D217, P24.335.w21)

It is important to consider the design of the transmitter and any means of attachment carefully to minimise potential adverse effects on the animal being tracked. (B379.38.w38, B380.3.w3)

  • For example, transmitters attached to flipper tags must be lightweight or they tend to tear out of the flipper. (B378.38.w38)
  • The biggest disadvantage of intra-abdominal transmitters is the requirement for anaesthesia and surgery to implant the transmitter. (B378.38.w38)
  • The total weight of the transmitter, including any attachment devices, should not exceed 2%, or preferably 1% of the body weight of the animal. (B23.3.w7)
  • Proper anaesthesia and aseptic surgical technique are essential when surgically implanting transmitters. (B23.3.w7)
Associated techniques linked from Wildpro

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Collection of Post-release Survival Data in the UK

  • The main organisation which collects data on ringed birds in the UK is the British Trust for Ornithology (BTO).
  • Data may also be collected and analysed by other groups, for example the South Devon Seabird Trust analyses data on the guillemots (Uria aalge - Common murre) which have been cleaned and released by the Trust. (D184)
  • It has been recommended that in order to improve data collection: (B370.4.w4)
    • All rehabilitated birds should be ringed, and if possible a national database of rehabilitated birds should be set up;
    • Information on the condition at capture, the treatment given to rehabilitated birds and on release variables (condition of the bird, and environmental conditions at the time of release) should be collected routinely to allow identification of the most successful cleaning and rehabilitation methods;
    • In addition to the standard BTO rings, colour rings should be used to increase rates of detection of live rehabilitated birds and recording of their presence at breeding colonies and their breeding performance, with ornithologists encouraged to search for such colour-ringed birds at colonies.


In the event of an oil spill, the Environment Group [if one has been set up] is responsible for collection of detailed records of: (D189.A2.w6)

  • all wildlife taken for cleaning and rehabilitation,
  • action taken,
  • survival rates in captivity,
  • release dates and locations,
  • ringing or marking of wildlife prior to release,
  • post-release survival success.


It is noted in the MCA's Scientific, Technical and Operational Guidance Note - STOp 1/2001 "Maritime Pollution Response in the UK The Environment Group" (D189) that:

For Birds:

  • "It is vital that numbers of all birds taken in by rehabilitation centres are recorded and their fate logged through the initial holding and eventual cleaning and release process.
  • It is vital that rehabilitated and released birds are ringed (with detailed records kept of their condition and ringing information) so that if they are subsequently found, they will not be attributed to a new pollution incident. This is also essential for increasing our understanding of the effectiveness of the rehabilitation process and the success of different methods of rehabilitation.
  • A protocol should be agreed between the RSPCA and EG [Environment Group] (or statutory nature conservation agency if no EG is established) over the release of cleaned birds to be rehabilitated. Release locations should be in areas where the risk is minimal, and where there are suitable food supplies nearby."


For Marine mammals:

  • "Agreed criteria and protocols for tagging / marking and release of rehabilitated marine mammals must be followed.
  • Animals must be released in the areas from which they were taken if possible, or elsewhere by agreement with the and EG (or statutory nature conservation agency if no EG [Environment Group] is established) with advice from the Sea Mammal Research Unit."


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Data from Post-Release Studies

"There is a growing body of evidence that clearly indicates survival rates are increasing for rehabilitated animals after release." (D210.6.w6)

Data from post-release studies varies widely in methodology and in interpretation. Some studies have taken a "worst case" approach and assumed that all released individuals not positively identified as alive and well after release, or found dead at a distance in time indicating prolonged survival after release, have died. Conversely other studies have taken a "best case" approach and assumed that all individuals, not either found dead or returned to rehabilitation within a short time of release, have survived. Obviously the interpretation of the same return data will vary considerably depending on which approach is used and it is probable that the true picture is somewhere in between. 

  • Each spill is different, involving different oil, different species, vastly different numbers of oiled casualties to be cleaned and rehabilitated, and different facilities, supplies and response capabilities. It is dangerous to extrapolate findings from previous spills to the likely effectiveness of future responses without considering all these factors as well as the ongoing improvements in response techniques and protocols. (J57.12.w1, P14.7.w16)
  • There are large variations between species in success rates for cleaning and rehabilitation. (D60.7.w7, D214.2.w2)
    • Variation in responses between species makes extrapolation between species on post-release survival impossible. (P14.7.w16)

The following variables may all affect the survival rate (during rehabilitation and post-release) of oiled birds:

  • The location of the spill; (P14.7.w16)
  • The type of oil; (B197.15.w15, D9, J4.181.w3, J29.8.w1, J59.30.w1, P14.7.w16)
  • Efficiency of response (J59.30.w1):
    • Whether a search and collection programme is undertaken, and if so, who by, and whether it is carried out effectively; (P14.7.w16)
    • The time oiled animals are in the field before being collected and their condition when brought in for rehabilitation; (D9, J4.181.w3, J312.16.w1)
    • The time span over which oiled wildlife casualties are retrieved; (D9, J29.8.w1)
    • The type of weather oiled birds are exposed to; (B197.15.w15, P14.7.w16)
    • Condition before oiling; (P14.7.w16)
    • Whether or not the birds are stabilised after capture; (P14.7.w16)
    • Whether birds are stable before being transported; (P14.7.w16)
    • Transport being timely; (P14.7.w16)
    • Availability of an appropriate facility; (B197.15.w15, B334.w1, D9, J4.181.w3, J29.8.w1, P14.7.w16)
    • Availability of trained, experienced personnel and whether trained personnel oversee all aspects of the response; (B197.15.w15, B334.w1, D9, J4.181.w3, J29.8.w1, P14.7.w16)
    •  If no facility was available, the length of time it takes for one to be found or constructed; (P14.7.w16)
    • Whether the facility has adequate water, and in particular sufficient hot water for washing oiled birds; (P14.7.w16)
    • Presence or absence of adequate ventilation in the rehabilitation facility; (P14.7.w16)
  • Species involved; (B334.w1, J29.8.w1, J59.30.w1, P14.7.w16)
  • Numbers of oiled wildlife casualties; (D9, J29.8.w1, P14.7.w16)
  • What product is used for oil removal; (P14.7.w16)
  • Whether blood parameters are checked before washing was allowed; (P14.7.w16)
  • Whether blood parameters are checked before birds were released. (P14.7.w16)


  • A 1996 analysis of bird ringing recoveries in North America concluded that most oiled, cleaned and released seabirds survived only a few days to weeks after release with median days survived of four to 11 days. It also concluded that there had been no improvement in post-release survival when comparing "recent" (birds cleaned after 1990) with those cleaned in previous years. For guillemots (Uria aalge - Common murre), 73 of 78 recoveries were within 60 days of release, with only four more than five months after release and only two of those more than a year after release (longest time between release and ring return was 919 days). Low long-term ring recovery rates were considered probably due to low survival. (J58.138.w1)
    • This study has been criticised, particularly for generalisations in its conclusions. It combined data from 13 species, involved birds oiled in various spills and used just 127 band returns to conclude that cleaning and releasing oiled birds was ineffective. (P14.7.w16)
  • A study of Pelecanus occidentalis californicus - Californian brown pelicans (Pelecanus occidentalis - Brown pelican) found that, compared with control birds, oiled and rehabilitated birds survived a shorter time, showed reduced movements, and did not join breeding colonies within two seasons after release. This was despite released rehabilitated birds having blood chemistry values within the normal range, normal behaviour and normal body mass at release, and despite 12 of 23 of the rehabilitated birds showing typical courtship and pairing behaviour in the one to two weeks before release. Rehabilitated birds showed high mortality during a stressful time: the moult followed by winter. There was some evidence that rehabilitated juveniles survived better than did rehabilitated adults. Transmitters were attached to the birds using neoprene straps, and weighed about 3% of bird body weight. [1996] (J313.32.w1)
  • A study of Uria aalge - Common murre (guillemots) in the UK concluded, based on ringing returns, that survival rates of oiled, rehabilitated guillemots was very low. The median survival time of oiled rehabilitated guillemots ringed since 1985 and recovered was seven days with a median distance moved prior to recovery of only 8 km. Survival rates were calculated as averaging 17% per 30 days in the first 60 days post release and 86% after that to the end of the first year post release, to give an annual survival of 0.6%: a survival rate of 0.7 to 1.3% of natural survival rates for the species. It was noted that there is some evidence that small numbers of oiled rehabilitated guillemots do survive and return to breeding colonies, and even rear young successfully. [1997](B370.3.w3)
    • It has been noted that ring returns may give excessive prominence to short-term losses if the areas close to the release point are heavily monitored [this gives an increased chance that birds surviving only a short time will be recovered, compared to those which survive for longer and are likely to die in areas less well monitored (thus not be returned)]. [1997](D214.2.w2)
    • There may be a bias in analysis when comparing rehabilitated guillemots released mainly in January to March (i.e. in winter) with unoiled birds ringed at colonies during the summer months. [1997](D214.2.w2)
  • Analysis by South Devon Seabird Trust of ring return data for guillemots (Uria aalge - Common murre) which had been cleaned and rehabilitated by the Trust, found (based on data to March 2003), that for guillemots ringed and released during 1993-1995, the mean time to ring recovery was 394 days and the mean distance travelled (i.e. distance from the Trust that the ringed bird was recovered) was 531 km. The data included 13 birds recovered after less than 35 days, of which 11 were found dead and two were found re-oiled (one having travelled 126 km by the time it was recaptured), with a further 15 birds recovered at 54 to 1585 days, of which six were re-oiled at 54 to 1,321 days, one was seen alive on Skomer, one was shot in France, one died in a sea trap in France and six were found dead after 105 to 1,408 days, including two found dead as part of a large wreck of seabirds in the North Sea. It was considered that those at liberty for more than 35 days before ringing recovery could definitely be considered to have been successfully rehabilitated, while some of those found at less than 35 days may have been successfully rehabilitated, based on distances travelled and/or time to ringing recovery. [2001](D184)
  • For about 3,000 guillemots (Uria aalge - Common murre) released after cleaning and rehabilitation at the West Hatch RSPCA centre, while 70% of ring returns occurred within two weeks, returns have also occurred after much longer periods, including several years. [1997](D214.2.w2)
  • Of 824 ringed guillemots (Uria aalge - Common murre) released by the South West Oiled Seabird Group, while 40 of the 62 returns occurred within two weeks of release, others occurred after much longer periods with ten at more than two years, one at more than three years, one at five years and two at more than nine years. [1997](D214.2.w2)
  • A radiotelemetry study of rehabilitated Uria aalge - Common murre following the Stuyvesant spill in northern California in 1999 used 8.5 g radiotransmitters, mounted using the subcutaneous anchor technique. A total of 31 rehabilitated murres had been found oiled, cleaned, rehabilitated, assessed as healthy, radio-marked and released within 17 to 21 days of capture (26 days for one bird), while 25 control birds were captured, radio-marked and released within 24 hours. The mean time for which oiled birds could be tracked was 61 days, maximum 142 days, compared with 80 and 145 days respectively for control birds. During the study period ten oiled and two control birds were known to have died. It was noted that these findings suggested better survival rate and survival duration than had been claimed in some previous studies. [2001](P60.1.w2) It was also noted that findings of blood samples taken before release suggested inflammation and secondary infections subsequent to petroleum oil exposure/captive care probably contributed to some of the deaths. Both mean tracking duration and survival (68%) were much higher than some previous studies on this species had suggested. It was acknowledged that oiled and rehabilitated individuals were four times less likely to survive than were control birds. Eighty percent of mortality in the oiled rehabilitated individuals occurred at 15 to 40 days post release and those which survived for forty days then appeared to show survival comparable to controls. [2003] (P14.7.w15)
  • A study compared 37 oiled, rehabilitated Fulica americana - American coots (Fulica - Coots (Genus)) with 38 control coots. All the birds were banded, fitted with numbered coloured neck collars, fitted with radio-transmitters (with mortality switches) and wing-clipped on one wing, then released into two marsh areas (20 rehabilitated and twenty control birds into an area of 0.56 ha land plus 0.46 ha water, the remainder into an area of 1.12 ha land plus 0.46 ha water), and supplementary food provided: i.e. the birds were soft-released. Birds were able to disperse from the enclosures when moulting replaced the clipped wing feathers with new complete feathers. Overall survival was significantly lower in the rehabilitated coots than in the control birds (49% versus 76%). Rehabilitated coots were noted to spend more time preening, to sleep less during the day, and to show feeding and drinking behaviours more frequently than controls (p <0.05). The rehabilitated birds initially lost weight in comparison to the controls but then recovered over one to two months. They showed significantly greater feather wear, completed their moult sooner, and dispersed from the study area sooner. It was noted that those rehabilitated coots which did not survive had failed to regain or maintain body condition. [2000](J316.107.w1, P59.1.w18)
  • A study compared health and blood parameters of oiled, rehabilitated Fulica americana - American coots (Fulica - Coots (Genus)) with control coots in a soft-release programme in which the coots were wing-clipped and released into large, fenced enclosures and blood sampled at 56, 81, 108 and 140 days. The greatest differences in blood parameters was noted in the 56-day samples. Oiled coots at 56 days had higher WBC count, albumin:globulin ration, and calcium concentration than control birds, and lower MCH, MCHC, creatine, total protein and globulin concentrations and alkaline phosphatase, alanine aminotransferase, and creatine kinase activities. Also at 56 days, oiled rehabilitated coots which subsequently died had significantly higher WBC count, PCV, total protein and globulin concentration, and lower albumin:globulin ratio, MCH, MCHC, glucose and sodium than control birds which survived. Oiled rehabilitated coots which subsequently died were found to have high chlorine (> MEQ/L) or cholesterol (> 449 mg/dL) concentrations at 56 days. Oiled rehabilitated coots which died had shown higher globulin and total protein levels than those which survived. From day 81 to day 140, the haematology and clinical biochemistry of oiled rehabilitated coots was not different from that of the control coots. Pathologically, oiled rehabilitated coots which died were emaciated (six of 11), with enlarged spleen (6/11) enlarged thyroids (6/11) enteritis (4/11), hepatitis (3/11) bile stasis (3/11), haemosiderin deposition in the liver (4/11), spleen (1/11) and kidneys (1/11), pododermatitis (3/11) and respiratory lesions including aspergillosis (3/11), granulomatous pneumonia (2/11) haemorrhage (2/11) and presence of parasitic granuloma (1/11). It was considered that inflammation, infection and iron utilisation or metabolism problems may have contributed to the deaths of those oiled rehabilitated coots which failed to survive. [2000](J316.107.w2)
  • A radiotelemetry study of Western gulls Larus occidentalis (Larus (Genus)) in California compared survival of oiled rehabilitated birds (oiled during a crude oil spill ("Torch/Platform Irene Pipeline" oil spill) with control non-oiled birds and with birds which had not been oiled, but had undergone the same rehabilitation procedures as oiled individuals. The cleaning and general rehabilitation of the gulls followed strict protocols carried out by professional personnel within the Oiled Wildlife Care Network. The study tracked birds for up to 235 days after release and found no difference in survival between the three groups (one death only, in a control bird), nor were there any statistical differences (P>0.05) in the size of the geographical areas used by the birds. Behaviour of individuals in the three groups (oiled rehabilitated, rehabilitated and control birds) were similar, as far as could be determined within the limitations of the study. Although transmitters were no longer working during the breeding season, data gathered while the batteries lasted indicated that the oiled rehabilitated gulls visited breeding colony areas "in a manner similar to control gulls" during both the winter and pre-breeding seasons. [20001 - 2003](J59.30.w1, P14.7.w18, P60.1.w1)
  • The same Melanitta nigra - Black scoters have been rehabilitated following oiling after successive spills in the same area (Waddensee) several years apart, indicating survival of the released birds following the first spill. [1997](P14.5.w8)
  • Eudyptula minor - Little penguins (Spheniscidae - Penguins (Family)) in Tasmania which had been cleaned and rehabilitated, were released 360 km or 540 km from their site of origin, because their habitat of origin was still oiled. Radiotracking confirmed that 65% of these birds returned to their site of origin within four months (this is a minimum estimate of the return rate, since transmitters had a battery life of only six weeks). [1998](J17.86.w1)
  • For Eudyptula minor - Little penguins (Spheniscidae - Penguins (Family)) oiled following the Iron Baron spill, northern Tasmania, Australia, minimum estimates of post-release survival, based on regular trapping, was 59% for penguins from North Island and 44% for penguins from Low Head; at both sites survival was significantly lower than for non-oiled penguins. Records showed that post-release survival could be predicted to the greatest extent by the extent of oiling at the time of capture, and the mass and body condition of oiled birds at the time of capture (which were affected by the extent of oiling); mass and condition at the time of release, and the sex of the birds, also affected survival, while the duration of time in rehabilitation, and whether or not the birds were translocated a distance (200 to 410 km) from their capture site for release did not affect survival. [2000](J47.27.w1)
  • A study over two breeding seasons after an oil spill, found that for Eudyptula minor - Little penguins (Spheniscidae - Penguins (Family)) in Tasmania, in the first breeding season oiled-rehabilitated females had a significantly lower success rate for fledging and egg success: 22% lower probability of oiled-rehabilitated females successfully fledging any chicks than for control nests or those with an oiled-rehabilitated male. In the second breeding season there was no significant difference in breeding success between oiled-rehabilitated and control birds. Chicks also had lower pre-fledging mass in both seasons if either parent was oiled-rehabilitated, which may reduce their survival rates. [1997](P14.5.w12)
  • Following oiling and rehabilitation of Jackass penguins (Spheniscus demersus - African Penguin) on St Croix Island, South Africa in July 1979, 87% of released rehabilitated penguins had been confirmed returned to the island  by February 1980. The penguins appeared normal, moulted as usual, returned to former nests and mates and six were confirmed to be breeding. [1980] (J313.11.w1)
  • Following the Apollo Sea oil spill near Dassen Island, off the coast of South Africa in 1994, in which 10,000 Spheniscus demersus - African Penguin were oiled, 4,076 of the 5,213 released penguins were flipper-banded. Of these, 2,652 had been positively resighted at breeding colonies within two years of release and by August 1996 the number re-sighted was still rising. Only a few penguins were recovered dead, including 37 within one year of release and a further 12 recoveries in the next 12 months; none provided evidence of death immediately following release. [1999](J58.141.w1)
  • Following the Apollo Sea oil spill near Dassen Island, off the coast of South Africa in 1994, in which 10,000 Spheniscus demersus - African Penguin were oiled, 2,962 penguins (nearly 73%) had been re-sighted back at breeding colonies. The number found dead has always remained below the expected reporting rate for non-oiled, non-rehabilitated birds and many birds are known to have survived, two, three, four, or five years after release. It was noted that, due to the problems in detecting and reading flipper bands, the numbers re-sighted should be considered minima of the total of survivors: after five years more penguins were still being detected for the first time after release. For individuals oiled in the Dyer Island spill, August 1995, about 40% had been re-sighted by two years post-release. For individuals oiled in the Cape Town Harbour spill, May 1998, by 16 months, 50% had been re-sighted (compared to 45% of the Apollo sea birds seen within one year of release). It was noted that a greater percentage of birds which were in adult plumage at the time of oiling were re-sighted than those which were in juvenile plumage when oiled. Analysis of elapsed time between banding and death for the oiled rehabilitated penguins indicated no significant differences in survival: oiled rehabilitated penguins survived as well as those which had never been oiled. Comparison of survival rates from live re-sightings using a program (MARK) which uses Likelihood Ratio Techniques to estimate survival between specified periods also found non-oiled and oiled rehabilitated birds to have similar survival rates. Data looking at oldest survivors also shows that oiled rehabilitated penguins are as likely to survive into their twenties as are those which have never been oiled. [2003](B334.w2)
  • Data from Jackass penguins (Spheniscus demersus - African Penguin) oiled in the Apollo Sea spill have shown that more than 60% of rehabilitated penguins re-sighted in the breeding colonies have attempted to breed, although a small proportion appear not to have attempted to breed. Possible reasons for non-breeding of some birds include effects of ingested oil on reproductive physiology, disruption of pair bonds and disruption of annual breeding cycles. Data on breeding success indicate that, for the first two years after oiling, when conditions (particularly feeding conditions) for breeding were good, oiled rehabilitated birds had similar breeding success to control birds, but that when conditions have been less favourable, or food in short supply, they were not as successful at raising chicks and growth rate depression was greater in chicks from rehabilitated birds during such periods than in chicks from control birds; there were no detected differences in chick rearing after the first two years. Other findings included disruption of the moult cycle in oiled rehabilitated birds during the first two years, after which synchronisation with non-oiled birds improved, and a slight disruption of the breeding cycle, with most oiled birds not attempting breeding until the second year after oiling. [2003](B334.w3)
  • Data from Jackass penguins (Spheniscus demersus - African Penguin) oiled in the Treasure spill in 2000, compared with those which were caught unoiled and temporarily relocated, showed that, for data to 17 April 2002, breeding of those subjected to precautionary capture was significantly less affected than that of oiled rehabilitated birds. This was probably due to less disruption to pair bonds and to breeding and moult cycles as well as the absence of toxic physiological effects of oil (there was also a probable small sampling bias since relocated birds were collected from a relatively small area and were probably more likely to be re-sighted). [2003](B334.w3)
  • In general, in is known that mute swans survive well following oiling and cleaning. [1998](B381.8.w8)
  • Mute swans (Cygnus olor - Mute swan) treated conservatively (supportive care only, not washed) following oiling with diesel during a river spill, showed significantly higher mortality than unoiled control birds during the first five months after release, with at least 33 of the 42 released birds (65%) still alive at that time. Thereafter, there was no difference in mortality (based on five years of monitoring). Only one oiled swan bred in the year following oiling but in subsequent years the swans bred normally and their productivity was normal. Swans had been released after 30 to 101 days (mean 63 days) in care and 42 of the original 51 birds were released (the other nine died in care). [1994](J321.94B.w1)
  • Following the Santa Clara River oil spill in 1991, in which 166 live birds were admitted for rehabilitation, and 123 birds were released, six bands were returned following shooting of the birds by hunters at 10 months to 3.9 years post-release. The birds concerned included two adult male Anas platyrhynchos - Mallard, two adult male and one adult female Anas americana - American wigeon (Mareca americana) and one adult male Anas strepera - Gadwall. Birds were recovered at 104.6 to 1,014.4 km distance from the release site. It was noted that the band return data did not provide information about variables such as whether or not the birds had bred successfully, although it was possible to determine that all the individuals had migrated successfully at least once after being released. An aggressive, rapid search and collection response and immediately available appropriate facilities for initial stabilisation and treatment, including washing, were considered likely to have contributed to successful rehabilitation. [1996](P62.14.w1)
  • Note: For most species, there is no data on rehabilitation rates. [1998](B381.8.w8)


  • A study on the post-release survival of North American river otters (Lontra canadensis - Canadian otter) used in a study on the effects of oil, found that the otters had lower survival rates than wild otters. Otters which were found dead from starvation, or that disappeared, relatively soon after release (21-106 days for the dead otters, 13 and 55 days for those that disappeared) had had relatively low haemoglobin levels pre-release. This may have played a role in the non-survival of these animals, since reduced haemoglobin levels can cause significant energetic costs and reduce capture of prey. The reduced survival rate of the experimental otters may have been related to poor food availability: the otters were released at the end of winter when schooling fishes were expected to enter the release area, but migration of fish into this area was delayed, which may have increased the length of time during which the otters had to cope with food shortages. [2002](J40.66.w3)
  • A study comparing the health status, diving behaviours, activity patterns, movements and survival of rehabilitated (all causes) harbor seal (Phoca vitulina - Common seal) (Phoca vitulina richardsi) pups with free-ranging pups, using head-mounted radio transmitters and in some pups dorsally mounted time-depth recorders, found that overall survival and behaviours were similar. One oiled pup was included in the study, and one tarred pup. Blood values at the time of release, behaviours post-release, and post-release survival, for the oiled and tarred pups were all similar to those of both wild (never rehabilitated) pups and those rehabilitated for reasons other than oiling. [2000](P14.5.w15, P60.1.w34)
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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