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Where do Great Blue Herons catch their snacks?

September 27, 2021

A picture of a great blue heron. The heron is standing on the edge of a pond.

Introduction

Great blue herons are part of the Ardea genus, which is a genus of birds that includes the herons, egrets, and bitterns. These birds are typically pretty large in size, with lengths usually greater than 80–100 cm. You’re most likely to find members of this genus in wetlands and other partially aquatic habitats.

Feeding Behavior

Great blue herons are carnivores, and they eat a variety of aquatic, semi-aquatic, and terrestrial prey. Their diet consists of predominantly fish, but they will eat pretty much anything that they can catch in their beaks. Fish are supplemented by amphibians, reptiles, small ground mammals, and even other birds. Great blue herons catch smaller prey in the jaws at the back of their beak, although they prefer to stab larger prey with their beaks.

There are 38 total feeding behaviors that have been recorded by members of the Ardea family. Each species in this genus uses multiple feeding methods — they’re creative hunters. Twelve of these strategies have been observed in great blue herons. The most commonly observed feeding behavior for the great blue heron is standing and waiting, where the heron simply stands still and waits for its prey to come to it. Herons also will move slowly, either through the water or through a field, in search of prey. More dramatically, the great blue heron will occasionally plunge-dive, diving down quickly and with force, to catch its prey. A particularly interesting feeding behavior of the great blue heron is to stick its bill in the water, and vibrate it. The underwater vibrations create ripples, attracting fish and other aquatic prey.

Habitat

Great blue herons prefer to live in wetlands, especially when nesting. Rather than nesting alone, great blue herons nest in colonies. Colony nesting is particularly common for birds that live in aquatic environments. Colonies can have over 500 nests (so easily over a thousand birds) all relatively close together. For large scale habitats and especially for colony nesting, great blue herons have preferences for certain types of habitats over others. The lacustrine wetlands is a particular favorite of Illinois great blue herons. Lacustrine wetlands are any wetland or deepwater area that has these three characteristics:

  1. Located in a valley/depression in the ground
  2. Lacking trees, shrubs, and other plant life that covers more than 30% of the area
  3. At least 20 acres in surface area

Biologists believe that herons may prefer this biome for their habitats due to the easy access to their preferred prey of small bodied vertebrates. Additionally, large wetlands tend to be isolated from human activity, which great blue herons tend to avoid if possible.

References

Combs, B. P. (2021). Great Blue Heron Forages for Fish by Bill-Vibrating. Western North American Naturalist, 81(2), 280–284. https://doi.org/10.3398/064.081.0213

Gibbs, J. P., & Kinkel, L. K. (1997). Determinants of the Size and Location of Great Blue Heron Colonies. Colonial Waterbirds, 20(1), 1–7. https://doi.org/10.2307/1521757

Knight, E. C., Vennesland, R. G., & Winchester, N. N. (2016). Importance of Proximity to Foraging Areas for the Pacific Great Blue Heron (Ardea herodias fannini) Nesting in a Developed Landscape. Waterbirds, 39(2), 165–174. https://doi.org/10.1675/063.039.0207

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About Author

Cassandra is a third year Computational Media student at Georgia Tech. Her concentrations are People and Interaction Design, and she hopes to work in UX/UI design after graduating from Tech. Outside of school, she enjoys tabletop RPGs, crochet, and photography.

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.