Wading Birds Pay Alligators For Protection By Sacrificing Chicks

Alligators act like bodyguards for wading birds, but their services don't come cheap. Orhan Cam/Shutterstock

Like cold-blooded mafiosos, alligators provide protection for wading birds in the Floridian Everglades in return for a high price, receiving payment in the form of chicks, according to a new study in the journal PLOS ONE.

Birds like herons, egrets, and storks have often been observed building their nests in close proximity to alligators as a means of staving off predators, which include animals such as raccoons and opossums. Since alligators are known to eat mammals of this size, yet cannot jump high enough to reach the birds’ nests (yes, alligators can jump!), researchers have hypothesized that some species of wading bird actively choose nesting sites above alligators. However, until now, little was known about what the reptiles themselves get out of running this scaly protection racket.

The study authors theorized that gators living in close proximity to bird colonies should be in better physical condition, suggesting two reasons for this. Firstly, they expected populations of prey animals to be more abundant in these regions due to a high availability of bird guano – or droppings – which provides a source of nutrition for many aquatic species.

More significantly, however, the researchers note that all species of wading bird are known to engage in brood reduction, whereby some chicks are ejected from the nest when birds lay more eggs than they can raise. These forsaken chicks are thought to provide a major source of extra food for the alligators, and represent the most obvious benefit for those living close to bird colonies.

To test this hypothesis, the study authors collected alligators from across the Everglades, comparing the physical and nutritional wellbeing of those living in colony locations to those from non-colony locations. This was achieved by taking small blood samples from each captured animal in order to analyze blood sugar levels and a range of other key indicators of nutritional health, known collectively as intermediary plasma metabolites (IPMs).

Herons and other wading birds often discard some chicks when their broods are too large. Wildnerdpix/Shutterstock

At the same time, they determined each alligator’s body condition by calculating the ratio between body mass and length. Known as Fulton’s condition factor, this figure has been used in several previous studies to indicate the overall health of alligators.

Interestingly, the researchers found no difference in IPMs between colony-associated gators and those not living close to bird colonies, yet noted a significant difference in Fulton’s condition factor. This disparity in body condition between the two sets of alligators was greater than the difference in body condition between birds capable of breeding and those incapable of doing so, as determined in a previous study. As such, the researchers claim that the extra nutrition obtained from wading bird chicks may be significant enough to affect the alligators’ reproductive success.

Explaining the lack of variation in IPMs between the two groups, the study authors note that none of the alligators in either group were completely starved of food, and therefore did not display IPMs associated with extreme starvation, regardless of their access to wading bird chicks. While this suggests that alligators lacking this component of their diet were not necessarily in dire need of food, the rest of the data clearly indicates that the overall nutritional health of these animals was inferior to that of colony-associated gators.

The researchers suggest that the experiment be repeated in other locations in order to verify these findings, and hypothesize that alligators may actively compete with one another for access to wading bird colonies, since the protection they provide comes with considerable benefits.

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