The effect of oil spraying on eggs of double-crested cormorants

  • Published source details Shonk K.A., Kevan S.D. & Weseloh D.V. (2004) The effect of oil spraying on eggs of double-crested cormorants. Environmentalist, 24, 119-124.


Numbers of double-crested cormorant Phalacrocorax auritus nests on the Great Lakes of North America have increased annually by about 28% from 1970 to 1991. The greatest increase (40%) was observed around Lake Ontario where in 1991 there were an estimated 9,190 nests but by 2000 this had risen to 24,500. Concerns were raised regarding this dramatic increase. Fisherman feared that the birds were causing a decline in fish numbers and conservationists considered that they might potentially impact on some endangered birds and rare plants. It was therefore considered that some form of population control was needed. Culling (shooting) adult birds is costly and may be unpopular with the public, therefore an alternative method was sought.

Spraying eggs with mineral oil has been used as a method to decrease the reproductive success of abundant avian species such as ring–billed gull Larus delawarensis, herring gull L.argentatus and Canada goose Branta canadensis. The oil clogs the pores on the egg shell and the embryo dies from asphyxiation. Therefore, a study was undertaken to determine if spraying oil on cormorant eggs might be an effective means of reducing reproductive output, and if application at different periods of the incubation satge affected hatching success.

Study site: The experiment was conducted on Farre Island situated in Hamilton Harbour, Lake Ontario, Canada. It is a man-made, circular (17 m diameter) rocky island with a single tree. It held 116 double-crested cormorant nests in 1997 when the experiment was undertaken.

Nest monitoring and tretments: Nests were monitored three times each week to establish lay date and clutch size. When an egg was laid, a numbered wooded stake was placed by the nest and the egg marked with a non-toxic marker pen. Clutch size was defined as the maximum number of eggs in a nest for three consecutive days. The usual clutch is 3-4 eggs and incubation period 25-29 days.

At the appearance of the second egg, the nest was randomly assigned one of five treatments:

1 - eggs oil sprayed in first week of incubation

2 - eggs oil sprayed in second week of incubation

3 - eggs oil sprayed in third week of incubation

4 - eggs sprayed with lake water in first week of incubation (control for handling of eggs)

5 - eggs not sprayed

The oil used was a 100% pure hydrocarbon oil (Daedol 50 NF oil) applied to the entire egg shell surface (about 7.5 ml/egg) using a garden spray bottle. Water was applied in the same way.

Nests were monitored for clutch size and hatching success for up to 10 weeks. Nest construction ability was used as a measure of breeding experience, younger birds (about 2-years old) tending to have poorly made nests and a smaller clutch (one or two eggs); such nests were excluded from the experiment. The expected hatch date was established from the date of the last egg laid in the clutch plus 29 days (the usual incubation period).

A total of 123 nests were recorded between 5 May to 25 July 1997. Of these seven were considered first-time breeders and were thus excluded. Therefore 116 nests were assigned to one of the five treatment groups. A comparison of treatment groups is shown in Table 1(attached). No significant difference was found in hatching success (range: 4-9%) between the oil spray treatments (1, 2 and 3). Hatching success was much higher (each 71%) for the two non-oil spray treatments.

Of those few eggs that hatched which had been oil-sprayed (1.4-4.4% of treated eggs), no abnormalities were noticed in any of the nestlings.

Conclusions: The oil spraying technique was 95-98% effective at preventing hatching. Hatching success was independent of the week of incubation that an egg was sprayed with oil. The maximum number of eggs in the colony occurred four weeks after the first egg was laid. This indicates that this would therefore possibly be the most efficient week to spray a colony to decrease hatching success. It could be a an effective method of double-crested cormorant control but further research is required to determine its effectiveness in reducing the incidences of second clutches.

Note: If using or referring to this published study, please read and quote the original paper.

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