An article which was published by The Condor, titled Effects of the 1982-83 El nino event on blue-footed and masked booby populations on Isla Daphne Major, Galapagos was written by Lisle Gibbs, Steven Latta and James Gibbs. They focus on the negative effects on the populations of these migratory birds created by the El Nino. These negative impacts include effects on the amount of available food, and reproduction.  The study included the measurement of three variables, “size of breeding populations, clutch size and degree of breeding synchrony, and the availability of nesting habitat for Blue footed Booby” (p440).  Oceanic disturbances like the El Nino have a large effect on the food supplies and breeding success of life such as the masked and blue footed boobies who rely on the ocean as their main source of food. Their method for determining the population of the blue and masked boobies was the count method.  Which is simply that they went to the various colonies of birds and counted the numbers of individual birds.  A bit of interesting information is that they determined the clutch size, brood size and the estimated age by simply looking at the plumage characteristic of the birds. They cite an auto named Nelson who provided the plumage characteristics.  The study found that there was a correlation between lower clutch sizes in years after the El Nino.

http://www.avibirds.com/pdf/M/Maskergent6.pdf