Hi Fabrizio - check out Rachel Buxton's recent publication on incidents of plastic fragments among burrow-nesting seabirds in New Zealand ( Marine Pollution Bulletin 74 (2013) 420–424) which contains a good range of earlier publication on this issue. Best wishes... Ursula
There are some recent and useful papers about this topic. I can recommend you one about plastic debris in the stomachs of dead Cory's Shearwaters from the Canary Islands, published by three colleagues (see the attachment).
There is also a paper on entanglement with marine debris by Northern Gannets (Morus bassanus), which may be of your interest. I attach the pdf file of it.
Here you find a 'plastic debris index' used for Shag nests: file:///C:/Documents%20and%20Settings/baccetti/Documenti/Downloads/OiseauxMarinsNicheursBretagne2010.pdf
Sorry, I did something wrong. I'm not attaching anything now, however the 2010 report is available at: http://www.observatoire-biodiversite-bretagne.fr/Espace-documentaire/rapports-d-actions/Rapport-Observatoire-des-oiseaux-marins-en-Bretagne
I attach the direct link to another publication on plastic debris in seabirds; see: https://www.researchgate.net/publication/259391870_Plastic_debris_in_Mediterranean_seabirds