Trophic cascades occur in nature when predators in a food web alter the behavior of their prey or suppress their number, thereby indirectly affecting the next lower trophic level in a positive way. This theory has been shown to be important in understanding the effects of removing a keystone species from a food web. Many studies in ecology have shown trophic cascades in aquatic and terrestrial ecosystems. A classic example of a terrestrial trophic cascade is the reintroduction of Gray Wolves to Yellowstone National Park, which reduced the number and behavior of Elk. This in turn released several plant species from predation of Elks and flourished across Yellowstone National Park.
This example of a trophic cascade is vividly shown and explained in a viral video “How Wolves Change Rivers.” However few trophic cascade have been done with bird predators. Such indirect effects involving raptors’ or aggressive birds are important in understanding trophic level changes as human activities, climate change, and other factors destroy or affect raptor nesting sites. An recent study done by researchers from the University of Arizona demonstrates the effect of trophic cascade with the Black Chinned Hummingbirds (Archilochus alexandri) breeding success when in association with Goshawk and Cooper Hawks (Accipiter gentilis & Accipiter cooperii). While Mexican Jays (Aphelocoma wollweberi), who prey on hummingbird eggs, alter their foraging behavior in presence of the occupied Hawks nest.
During the course of three nesting seasons researchers documented the placement of hummingbirds nests in relation to the location of 12 different hawks nest. Failure of a hawks nest, due to a predator, provided a test that was measurable of hummingbirds success as well as the foraging traits of jays. Their study of 342 hummingbird nests showed two important patterns for hummingbirds. One was that 80% of hummingbirds preferred to nested nearby a active hawks nest while 20% of hummingbirds nested near an inactive hawks nest.
The seconds is that out of 80% of hummingbirds who chose to nest nearby a active hawks nest had a higher chance of survival (31%) than the 20% who chose an inactive hawks nest (6%). This idea was further reinforced when within 2 weeks the survivability of hummingbirds dropped to almost 0% when 4 hawks nest failed. The foraging behavior of the jays was heavily impacted in reaction of a active hawks nest. The closer they came to the hawks nest the higher they flew. That is because the hawks hunt by nose diving from perches within the canopy.
Therefore the higher the jays flew the safer they were from the attacks of hawks. This cave the hummingbirds a safe area in the shape of a cone. However when the jays learned the hawks nests were inactive or destroyed they became much more daring in foraging for hummingbird eggs. (Greeney et al, 2015)
Another example of an aerial trophic cascade was done by researchers from Simon Fraser University showed Great Blue Heron (Ardea herodias fannini) nested nearby Bald Eagles (Haliaeetus leucocephalus) to gain predator protection. Where 70.8% of 1165 nested within 200 meters of 3 active Bald Eagle nest. This is because the Bald Eagle provided the most protection within 200 meters. The 70.8% of herons that nested nearby the Bald Eagle had a highest nesting success than those far from the site.
The researchers observed that the Great Blue Herons alarm calling alerted the resident Bald Eagles to presence of other eagles. The residents would respond to the Great Blue Heron alarm calls 57.4% in order to defend their own nest sites during breeding seasons. The Great Blue Heron benefits the Bald Eagles by helping defend their territory. meaning that the colony has to contain at least 39-58 active nests for the gain in nest success to outweigh losses to the guardian eagle. The three eagle-associated colonies had 202, 222, and 400 nests, while the three largest non-eagle associated colonies had 101, 72, and 46 nests. (Jones et al, 2013)
A study done by Alfred Trnka and Tomas Grin showed Little Bitterns had less nest intrusions when partenered with Great Reed Warblers. That is because Great Reed Warblers showed less aggressive behaviour (~5x less) towards neighboring Little Bitterns than solitary Little Binterns but had the same amount of aggression towards the Turtle Dove. Within 40 meters of an active Great Weed Warbler nest the amount of attack dropped 51% and the breeding success increased to 88%. As the Little Bitterns got closer they breeding success increased even more. (Grim et al, 2014)
In conclusion nest predation is a major source of reproductive failure. We know that bird parents can assess and test experimentally reduced nest predation risk and alter their reproductive strategies. The University Of Montana shown this by experimentally reducing nest predation risk. When in safer environments parents increased investment in young through increased egg size, clutch mass, and the rate they fed nestlings. Demonstrating that birds can assess nest predation risk at large and that nest predation plays a key role in the expression of avian reproductive strategies (Fontaine et al, 2006).
So the various trophic cascades experiments did not happen by fluke. It was because the breeding birds assessed their reproductive rate and chose to nest nearby favorable raptors and aggressive birds. They knew that in doing so would be more favorable than being solitary as those nests are quick to fail. While the predators, such as the Mexican Jays, changed their behavior in response to the dangerous threat.
