3-25 PPM in gulls, terns, cormorants, mergansers, herons, and ospreys

I think this would be hard to quantify for a lot of reasons (the main one being that many of these birds migrate and are long lived, and therefore aren't feeding on a specific single fish population), but to find an approximate answer you could try working backwards. Although it varies highly among species, 3 ppm seems to be the threshold level in eggs at which egg thinning occurs in highly sensitive species, and DDT levels in adult birds of over 50 ppm leads to eggs with 2-3 ppm. So consuming what amount of fish with x ppm DDT would lead to a bird having over 50 ppm DDT? It would be highly species dependent because it would depend on the longevity and feeding habits of both predator and prey. If you can find studies that give real values for DDT found in wild fish populations, you would have some numbers to plug in for x. You would also have to change your egg and adult bird thresholds depending on which species you're focusing on. This wouldn't give you a definitive threshold, but that's how I would approach thinking about this problem if the exact data you are looking for aren't available.

Thanks for the responses. The reason I posted the question is that I work with a very small threatened population (

Since DDT biomagnifies, I would imagine there's a good chance concentrations are high enough in the terns to cause eggshell thinning if you're seeing concentrations in fish as high as 26 ppm. DDT can increase in concentration 10 fold from one trophic level to the next.

You might also consider shortage of Ca supply/availibilty due to environmental conditions as a possible cause of eggshell thinning.