This is a fascinating topic.
I have observed the following behaviors in waterfowl and shorebirds - most species have a tendency to form mixed flocks in feeding situations as well as in migratory and roosting situations. However, there are species such as American Woodcock that do not seem to socially interact with other bird species (although this species is difficult to observe frequently and for long duration), but they certainly are unlikely to intermingle with any other species of shorebird/wader. You also see large rookeries of herons and ibis, as well as mixed nesting site selection of shorebirds/waders up on the northern tundra, as well as mixed gull and tern colonies on marshlands. Each species in the mix provides SOMETHING of service to the other species involved in the congregation. Sometimes it is much more clear than others what the delegation of roles among species are. For example, back home in New Jersey out on the saltmarshes during the breeding season you have mixed nesting colonies of Laughing gulls, Forster's Terns and Common Terns. At my local patch in the far southern part of New Jersey in Cape May Co, Common Terns are the more abundant nesting species as compared to Forster's Tern, which is the opposite of most of the New Jersey marshland habitats. Within these mixed colonies, the Common Terns nest around the perimeter of Laughing Gull colonies pretty much exclusively. The Terns are very aggressive, more so than the Forster's Terns. The Laughing Gulls seem to be the 'stronghold' of their city whereas the Terns are sort of the defenders of the castle wall. Upon deterring an intruder, the terns will venture out away from the nests to ward off intruders before they get close, but in the event that a predator were to approach closely, the larger and more powerful laughing gulls are prepared to defend as well. The combination of these deterrents is far more effective than one method without the other for pretty intuitive reasons. This is certainly a very prevalent intra species social gathering. You will see terns and gulls interacting directly, often squabbling with one another, and Laughing Gulls will even very rarely eat unattended Common Tern chicks. I wouldn't really say that they get along, but they leave each other be for the most part, presumably unless territory boundaries are tested, which happens pretty regularly being how close they are packed together.
A few years ago, we even had a very strange occurrence of a beach nesting species, Royal Tern, nesting and laying an egg amongst a mixed Common Tern x Laughing Gull colony. To my knowledge this is possibly undocumented and at best extremely rarely observed. Common Terns will mostly nest on the beaches of New Jersey and NOT in the marshes, which creates an unusual opportunity for study in the southern part of Cape May county, and gets even more interesting with the rare introduction of a Royal Tern into the marshland colonies. This is perhaps one of the only places where this even could happen, where a Royal Tern would nest in a mixed Laughing Gull colony, I am really not sure what the breeding status and distribution of Common Terns is like elsewhere to the south of Cape May. Cape May is about as far north as Royal Tern has been recorded breeding to my knowledge. Make no mistake, the Laughing Gull is the king species of the colony. Like I said, the stronghold, they get the highest ground, or the most protected interior sections of the colony as compared to the terns.
There was also a case where I observed a family group of banded Red Knots on the beach in Brigantine, NJ. This family group had been banded together on the same date 6 years prior to my resighting of their leg bands. In total there were approximately 10 birds in a flock together. Of those 10 birds, 3 were banded 6 years prior together on the same date at the same location, indicating that they had formed a long-term migratory travel family group.
Furthermore, it has become apparent to me through observational experience that the best strategy that I have employed to find a migrant Golden-winged Warbler or a hybrid is to find a Blue-winged Warbler and just keep diligently looking around its vicinity. Likewise, to find a hybrid backcross, if you find a single Golden-winged or Blue-winged migrant be diligent and keep searching the vicinity closely, and the inverse is true as well in the aiding of finding a hybrid backcross. These birds also seem as if they are extremely likely to travel in migratory family flock.
Last edited by tom baxter : Friday 10th January 2020 at 06:41.