The northwest of Mexico is part of one of three migratory corridors, the Pacific corridor, in the American continent. It is considered one of the most important regions for shorebirds, receiving more than one million each winter.
A few years ago, in the Upper Gulf of California and Colorado River delta Biosphere Reserve, the team observed a significant number of Red Knots (Calidris canutus roselaari) on the muddy shore, which raised their interest in starting to study more about their use of the area.
Beginning in 2010, a large group of about 800 Red Knots was found, but this time on a sandy shore in the Gulf of Santa Clara, Sonora. This caused researchers and civil society associations to observe with greater detail the passage of the bird through the Gulf of Santa Clara. In general –during the non-breeding season– shorebirds prefer muddy environments as their diet is based on invertebrates like polychaete worms, clams, and small snails, which are more abundant on these types of beaches.
Adriana Hernandez Alvarez, a Pronatura Noroeste field technician, explained that the results of these observations determined that shorebirds were feeding on the sandy beach, so in addition to monitoring the area, researchers began to investigate what the birds ate.
When analyzing a sample of sand in the laboratory, the team found eggs of the Gulf Grunion (Leuresthes sardina), an endemic fish that crawls with the tide to spawn and fertilize eggs on high tide days. “The object of study is the birds, but to understand some changes in their behavior, distribution, and abundance, we also needed to understand what was happening with the fish,” she said.
The research conducted by Pronatura Noroeste and the UABCS Bird Laboratory shows the interrelation between the migration of one species and the reproductive stage of another and identified that from the middle of February to the first days of May, the Red Knot feeds mainly on the Gulf Grunion spawning. In an average period of 20 days, the birds gain around 40 grams, approximately 30% of their body mass (as if a 70 kg person gained 21 kg in 20 days), allowing them to continue their journey toward Alaska and Russia for the breeding season.
“We are interested in knowing about this population of fish, its current status, and how it is changing over time, [since] as with any other predator-prey relationship, both populations are codependent. The predator depends on the availability of this prey, just as the population of prey also depends on the predators,” said Hernandez Alvarez.
The only available information dates back to the 70s; therefore, during the spawning season, the team performs research that includes measuring variables like weight, length, or density of fish to analyze the current population and be able to compare this information with that already in existence.
The researcher pointed out that the availability of Gulf Grunion eggs on the beach can affect the reproductive success of the birds. If they leave the area with a lower body mass, they reach the breeding site at a lower weight and do not have the energy to find a partner, lay eggs with thinner shells, or have weak offspring. This is why it is essential to continue monitoring birds and fish, to add more data that will allow the detection of changes in the population, since this area – in which the Gulf grunion reproduces – is under high pressure from anthropogenic factors, “We are talking about a species that is facing threats right at the most vulnerable moment of its lifecycle,” said Hernandez Alvarez.
To continue establishing the relationship, presence, and distribution of birds with respect to the spawning of Gulf grunion, the team plans to improve the capture method for 2019 monitoring, with which they expect to identify the variation of their birds’ mass in their weight and establish its relationship with the density of Gulf Grunion eggs.
