This feature provides an example of the way a small sub-group of birds, found in the same habitat as a larger group of its relatives, have evolved, avoiding competition with each other. It also illustrates how two species within the small sub-group have developed to live together in particular location.
Family Ardeidae contains 65 species variously known as Herons, Egrets and Bitterns. They range from large and confident, to medium sized and secretive. Seven species, called Night Herons, have established niches where they avoid competition from larger birds by being nocturnal feeders.
The most well known is the Black-crowned Night Heron which has a worldwide distribution. Other species are the White-eared, Japanese and Malayan found in Eurasia, White-backed in Africa, Rufous in Australia and Yellow-crowned in North and Latin America.
The two species which interest me most are the Black-crowned and the Yellow-crowned Night Herons and for me the best place to see them is Stone Harbor Wetlands in Cape May County, New Jersey, USA. I have seen and photographed them for myself in this habitat but my colleague and website member Nelson can be relied upon to photograph them almost on request. I have used various images from Nelson's Page in http://wgbwcopy.wikidot.com to illustrate this feature.
Although the BCNH and YCNH are distinctly different species they are not always easy to tell apart. They are often seen resting together in high branches of trees during the day but this is as close as they get.
As soon as we start to look at feeding techniques and general behaviour we can see the way these two species have established their own niches. This is illustrated the following gallery of images and the comments below. Click on each image to see a larger view and use the NEXT and PREVIOUS buttons within the image to scroll forwards or backwards. Images are numbered starting top left and reading left to right.
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In images 1 and 2 the two uppermost birds are YCNH's and the two lower birds are BCNH'. The main characteristic which differentiates the species is the black crown of the BCNH and the white cheek below the eye of the YCNH. Both species are resting quite happily, close together in the same tree.
Images 3 and 4 show the BCNH where it is likely to be when not at rest. They wait patiently in dense vegetation close to the waters edge watching for fish and other prey. They tend to be at their most active at night.
Images 1 to 5 were taken by Nelson in Stone Harbor. Image 6 is one of my images, taken in Hungary, included because it shows the white, wispy head trailers which are sometimes a feature of Herons and Egrets.
Images 6,7 & 8 are YCNH taken by Nelson. Although they are still nocturnal feeders they are much more likely (than BCNH) to be seen standing on mud-flats, river banks or even posts in the water, when they are at Stone Harbor. Where the BCNH is a patient forager the YCNH is a specialised feeder, often on the move, walking along searching for crustaceans and crabs.
Image 7 shows an adult YCNH with a juvenile to its right. Image 8 shows two adults "on sentry duty" with an Osprey flying overhead. Image 9 shows an adult with black head trailers.
The behaviour of the Yellow-crowned Night Heron is completely at variance with that of other Night Herons. Its specialised feeding
probably explains why it has not expanded its distribution outside Latin America.