Petrels, part of the order Procellariiformes, are a diverse group of specialized seabirds known for their long-distance, pelagic lifestyle. Characterized by their unique tube-noses, which house specialized olfactory organs, these birds navigate the vast, open ocean in search of sustenance. Their diet and feeding behaviors are finely tuned adaptations to the patchy and unpredictable distribution of food resources in the marine environment. This article explores the prey items that fuel these oceanic travelers and the techniques they employ to capture food.
Primary Prey and Nutritional Needs
The petrel diet is primarily composed of organisms rich in fat and protein, a necessary energy source for their extensive, energy-intensive flight over the ocean. Crustaceans form a foundational part of the diet for many species, particularly smaller petrels and prions that target organisms like krill, copepods, and amphipods.
Cephalopods, primarily squid, are also a major food source, particularly for larger petrel species and shearwaters. Many squid species, along with small schooling fish known as myctophids, migrate vertically to the surface at night, which is exploited by nocturnal-feeding petrels. Adult petrels convert partially digested food into a concentrated, energy-rich stomach oil to feed their chicks. This oil, high in lipids, is easily transportable over long distances and allows for rapid growth and energy maintenance in the young birds.
Diverse Foraging Strategies
To capture their varied prey, petrels have evolved specialized hunting techniques that minimize energy expenditure. A fundamental method is surface seizing, where a bird snatches prey from just below the surface layer, often while sitting on the water. Storm petrels frequently utilize a unique variation, pattering their feet on the water while hovering or gliding to pick up small morsels.
Another common technique is dipping, which involves flying low and briefly contacting the water to snatch a prey item without fully landing. Gadfly petrels often use this method to capture mesopelagic prey that has risen to the top layer of the water column. Other species, notably shearwaters and diving petrels, employ pursuit plunging, involving a flying dive into the water and swimming underwater using their wings for propulsion.
The highly developed sense of smell guides petrels to widely dispersed food patches across the ocean. Petrels possess some of the largest olfactory bulbs among all birds, allowing them to detect specific scent compounds released during the feeding process. One such compound is Dimethyl sulphide (DMS), which is released when zooplankton grazes on phytoplankton, signaling an area of high productivity and concentrated food.
Influence of Marine Environments and Human Activity
The distribution of petrel food is heavily influenced by dynamic oceanographic processes, forcing the birds to be highly nomadic in their search for sustenance. Upwelling zones, where nutrient-rich water rises to the surface, and ocean fronts often concentrate planktonic organisms and small schooling fish, creating reliable foraging hotspots. The presence of subsurface predators, such as tuna or marine mammals, also plays a role by driving prey to the surface where petrels can easily access them.
Opportunistic feeding is a significant component of the petrel diet, particularly through scavenging. Many petrel species readily consume carrion, including the remains of dead whales or seals, along with discarded fish waste. This behavior has led to a strong association between petrels and commercial fishing vessels.
Modern fishing practices have inadvertently created a new, concentrated food source in the form of discarded by-catch and offal, which is readily exploited by scavenging petrels and fulmars. This discarded material, often consisting of demersal fish or parts of large species, can represent a substantial part of the diet for some populations. While this provides a reliable food source, it also brings the birds into direct conflict with fishing gear, leading to incidental mortality.