Sharkbites Saturday

Birds of a feather are eaten together: Young tiger sharks take a bite out of migrating songbirds

What’s the deal with tiger sharks?

Tiger sharks occur globally in tropical and sub-tropical regions of the ocean. A large shark with a flexible appetite, they commonly reach 16 feet in length, though they can grow past 24 feet. Normally keeping to coastal areas where they feed on fish, turtles, and the occasional seabird, tiger sharks have been known to make excursions to remote islands. They cross oceans to other coasts in order to feed on abundances of prey, including nesting sea turtles and seabirds. These sharks have also been observed moving great distances to scavenge on large carcasses like whales.

Photo of a tiger shark by AlKok and licensed under CC BY-NC-SA 2.0.

Despite their resiliency to changing food sources, the International Union for the Conservation of Nature’s Red List of Endangered Species lists the shark as “Near Threatened.” This is due mainly to their long lifespan and slow reproduction, which is common to most large sharks, as well as over-fishing for the shark fin trade.

Although they can reach lengths over 12 feet at maturity, young tiger sharks are around one to three feet long at birth and need time to learn efficient hunting and feeding behaviors. This likely makes their feeding even more opportunistic than the adults, and a team of researchers is beginning to discover just how broad the young tiger shark menu is.

Land Birds and Gulf Sharks

Spring and autumn in many parts of North America are marked by vast migrations of songbirds. In autumn, many species travel south to wintering ground in the southern US, Central, and even South America, while spring sees their return to the northern portions of their range. Many of the birds are funneled through the Gulf of Mexico, directly over the nurseries and habitats of tiger sharks. Storms, exhaustion, and a miscalculation of their course can all land a migrating songbird in shark waters. Evidence exists from many parts of the world that tiger sharks occasionally feed on seabirds, but recordings of songbird depredation in the scientific literature are scarce. However, during a longstanding shark survey in the northern Gulf of Mexico in 2010, a captured juvenile tiger shark regurgitated the feathers of a songbird, sparking an investigation into just how important songbirds are to the diets of these sharks.

Diagram showing how migrating songbirds wind up as meals for opportunistic sharks. Diagram by Tyler Plum.

Even sharks like chicken

The team set out to determine just how often the sharks are capitalizing on this abundant food source. Over the course of nine years the team surveyed the stomachs of 105 tiger sharks during annual shark surveys in the northern Gulf of Mexico. In order to determine if songbirds were present in the sharks’ stomachs, the team used gastric lavage, a technique were water is pumped into the stomach to make the sharks regurgitate their food. DNA markers from bird tissues found in the stomachs were used to determine which species were eaten. The team then went further and used eBird, an electronic citizen science-driven website that records bird sightings, to compare the abundance of local sightings of those species that were found in the shark stomachs.

Songbirds: Fast-food or Favorite Meal?

The team found that of the 105 sharks sampled, 41 (39%) had eaten a songbird! The DNA tests revealed that the sharks had eaten 11 species of songbird (see the chart below). Oddly, no seabirds were detected in the stomachs. Of those 41 sharks that had eaten a songbird, almost half of them had been young tiger sharks.

Each of these species were found in the stomachs of tiger sharks! These photos by Unknown Author are licensed under CC BY-ND, CC BY-SA-NC, or CC BY-ND.

While this is not conclusive evidence that tiger sharks are targeting songbirds during these annual migrations, the authors believe that this extra food source may be occurring at a crucial time in a shark’s life cycle, specifically before they learn to feed effectively on their own. The authors also note that tiger sharks change their nursery grounds to coincide with areas that have a high abundance of prey for young sharks, and they believe it may be one factor that helps explain why the northern Gulf of Mexico contains such large numbers of juvenile tiger sharks.

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