WI Cormorant Research Group Study Cormorant databases - Great cormorant created on 08-02-2001

Eggshell thickness measurements of Cormorant eggs:
methods and some backgrounds


Sjoerd Dirksen & Theo Boudewijn

Bureau Waardenburg bv
P.O. Box 365, 4100 AJ Culemborg, The Netherlands

1. Introduction
One of the negative effects of organochlorine contaminants is the thinning of eggshells of birds. This has been found in several species of fish-eating waterbirds (Anderson et al. 1969, Anderson &. Hickey 1972, Koeman et al. 1972, Cooke 1979, Pearce et al. 1979) and raptors (Ratcliffe 1967, 1970). In some cases it has caused severe reductions in reproductive success.

In most cases eggshell thinning is caused by p,p'-DDE, a persistent metabolite of the well-known insecticide DDT. Several authors have presented dose-effect relations between levels of p,p'-DDE in the egg and eggshell thickness (e.g. Koeman et al. 1972). The relation between p,p'-DDE and eggshell thinning has been established at a molecular level as well (Lundholm 1987).

In this short document we present some information on the methods we have used to do eggshell measurements in research on effects of organochlorine contaminants on Cormorants (Phalacrocorax carbo sinensis) (Dirksen et al. 1991).

2. Field sampling
Damaged eggs as well as the shells of hatched eggs can be found under the nests of Cormorants. It is important to collect these eggshells throughout the whole breeding season. In the first weeks mainly eggs that have failed to hatch can be found, later on the shells of hatched eggs - form the majority. Every egg should be packed and labelled separately. Normally it can be seen from the egg remains whether or not a young has hatched from it. This information should be documented as well.

3. Preparing the eggshells
The eggshells have to be cleaned from mud, faeces, yolk remains etc. It is sometimes difficult to discriminate between faeces and the vaterite layer that Cormorants often have on (parts of) their eggs. The faeces always come off easily in lukewarm water, while the vaterite can only be removed with a little force. The vaterite should not be removed. Furthermore, the membranes on the inside have to be removed. This is necessary because of the fact that the membranes of hatched eggs often come off. In order to be able to compare all eggs with each other, they all have to be measured without membranes. The membranes can easily be removed in lukewarm water, using a tooth-brush or other soft brush.

For every egg the following data are recorded while preparing the eggshell:

- sample number;
- sample date;
- nest or clutch number (for combination with breeding success data);
- part of the eggshell that was sampled (see Figure 1 );
- part of the eggshell where pieces are taken for measurements;
- presence of vaterite on the outside of the egg (0: none; 1: 25% coverage; 2: 50%; 3: 75%; 4: 100%);
- hatching result of the egg: hatched, probably hatched, not hatched or probably not hatched;

4. Measuring eggshell thickness
From the equator of the cleaned egg three little pieces are taken for the measurement. These are air-dried. Parts covered with vaterite are avoided for the measurements. The vaterite can cause great variation in thickness. However, it only contributes very little to the prevention of breaking (Lundholm 1987), which justifies this choice.

On all three pieces of the eggshell three measurements are taken. We use a mechanical micrometer, accurate to 0,001 mm. The nine measurements are averaged to get a mean figure for the egg.

5. Comparison with eggs from museum collections
Eggs in museum collections normally still have membranes, as they were collected as more or less freshly laid eggs. In order to be able to compare the eggshell thickness data obtained as explained above, it is necessary to establish the thickness of the membranes. This can be done by measuring some of the pieces from the field samples with membranes, removing the membranes and repeating the measurement.

6. Why measure on the equator of the egg?

In order to check whether eggshell and membrane thickness vary along the egg, eight eggs were measured on seven different places between the top and the bottom of the egg (
Figure 1). We found significant differences between some of these measurements, and therefore decided to choose one fixed place to measure. The equator was chosen, following other researchers (Koeman et al. 1972, 1973, Newton 1979), and because the thickness of the areas nearby (3 and 5, Figure 1) did not differ significantly from the equator.


- Anderson, D.W., J.J. Hickey, R.W. Risebrough, D.F. Hughes, & R.E. Christensen 1969. Significance of chlorinated hydrocarbon residues to breeding pelicans and cormorants. Can. Field Nat., 83: 91-112.

- Anderson, D.W. & J.J. Hickey 1972. Eggshell changes in certain North American birds. Proc. XVth Int. Orn. Congress, 514-540.

- Cooke, A.S. 1979. Egg shell characteristics of Gannets Sula bassana, Shags Phalacrocorax aristotelis and Great Black-backed Gulls Larus marinus exposed to DDE and other environmental pollutants. Environ. Pollut., 19: 47-65.

- Dirksen S., T.J. Boudewijn, L.K. Slager & R.G. Mes 1991. Breeding success of Cormorants (Phalacrocorax carbo sinensis) in relation to the contamination of their feeding grounds. pp. 232-243 in: M.R. van Eerden & M. Zijlstra (eds.). Proceedings workshop 1989 on Cormorants Phalacrocorax carbo. Rijkswaterstaat Directie Flevoland, Lelystad.

- Koeman, J.H., T. Bothof, R. de Vries, H. van Velzen-Blad & J.G. de Vos 1972. The impact of persistent pollutants on piscivorous and molluscivorous birds. TNO-nieuws, 27(10): 561-569.

- Lundholm, E. 1987. Thinning of eggshells in birds by DDE: mode of action on the eggshell gland. Comp. Biochem. Physiol., 88C, 1-22.

- Pearce, P.A., D.B. Peakall & L.M. Reynolds 1979. Shell thinning and residues of organochlorines and mercury in seabird eggs, Eastern Canada, 1970-76. Pestic. Monit. J., 13: 61-68.

- Ratcliffe, D.A. 1967. Decrease in eggshell weight in certain birds of prey. Nature, 215: 208-210.

- Ratcliffe, D.A. 1970. Changes attributable to pesticides in egg breakage frequency and eggshell thickness in some British birds. J. Appl. Ecol., 7: 67-115.