Breeding ecology of the Red-billed Tropicbird Phaethon aethereus under contrasting environmental conditions in the Gulf of California

We studied the breeding ecology of Red-billed Tropicbirds Phaethon aethereus at Farallon de San Ignacio, Sinaloa, in the south-central part of the Gulf of California, during two years with different oceanographic conditions: 2004, a neutral year, and 2007, a moderate El Nino year. We characterized oceanic changes by sea surface temperature and chlorophyll a concentration (150 km around the colony) from monthly satellite images. During two 5-day visits every month from January to May of each year we recorded timing of breeding, parental attendance, chick diet, and maximum diving depth of adults. We analyzed hatching and fledging success, chick growth and body condition. In agreement with oceanographic differences, we expected differences in timing of breeding, diet and foraging behaviour and lower breeding success and chick growth in the El Ninio year. Sea surface temperature was higher, and chlorophyll concentration lower during early 2007 than in 2004. Average laying date and peak of laying occurred later in 2004 than in 2007. In 2007 adults dived deeper (2.09 +/- 0.96 vs. 0.96 +/- 0.66 m), had a different and more varied diet, and spent more time at sea at the expense of nest attendance than in 2004, suggesting that less food was available for this species in 2007. In agreement, hatching success was lower (35 vs. 75%), and chicks were lighter (641 vs. 739 g) and in worse body condition in 2007. Overall, despite the lower body condition of the chicks, the species seemed to exhibit some capacity to cope with the different conditions derived from warmer waters during the mild El Nino of 2006-2007.