The government of Peru is investigating the deaths of more than 500 pelicans along a 70km (40-mile) stretch of the country's northern coast.
Officials say most appeared to have died on shore over the past few days.
Scientists have also found the carcasses of 54 boobies, several sea lions and a turtle.
They were found in the same region where some 800 dolphins washed ashore earlier this year. The cause of their death is still being investigated.
The Peruvian government said it was "deeply worried".
A preliminary report said that there was no evidence to show the pelicans had died at sea, but rather on the beach where they were found.
But it said further tests would be needed to establish the cause of death.
The Peruvian Maritime Institute (Imarpe) said so far 538 dead pelicans and 54 boobies had been found in various stages of decomposition, although most appeared to have died recently.
In addition, five badly decomposed sea lions and a turtle carcass had been found on shore, Imarpe said.
Local media reports suggest more than 1,200 dead pelicans have been found in the Piura and Lambayeque regions.
Between January and April of this year, some 800 dead dolphins washed ashore in Lambayeque, according to government figures.
Peru's Deputy Minister for Natural Resource Development, Gabriel Quijandria Acosta, said a virus might have killed the dolphins.
A viral epidemic outbreak was linked to similar deaths of marine wildlife in Peru in the past, as well as in Mexico and the United States.
Analysis on the dolphins so far suggested they had contracted a morbillivirus, which belongs to the same group as the measles virus in humans, Stefan Austermuehle of a local NGO, Mundo Azul, told the BBC.
"We know that in other cases in the United States up to 50% of populations were killed by the virus," he said.
"What we also know...is that in previous cases animals that have higher loads of pollutants in their body will fall easier victims to these kind of diseases because their immune system is weakened."
Imarpe scientists said results of tests carried out on the dead dolphins would be released in the coming days.