Mixed Species Foraging in Birds

Birds of a Feather [1]

"Sociability is as much a law of nature as mutual struggle." Pëtr Kropotkin [2]

In Mutual Aid: A Factor of Evolution, [2] Kropotkin only mentions cooperation among members of the same species. While looking into the phenomena of the Dawn Chorus, I learned that many species of birds sang together at dawn. I found little research except for different starting times, culminating with everyone singing up to an abrupt stop at dawn. [3] Upon completing the article I discovered that birds also foraged in mixed flocks. I found this astonishing, animal behavior is so diverse and any theoretical explanations are still in their infancy. There are several things to be noted about behavior. First, is that a behavior has a life history. There is a starting point or birth. Next, a period of development to maturity, followed by senescence to an end. The factors that lead to a behavior starting, its maintenance, and its ending, are open questions. Another open question is how it evolved in the first place. The reason for the evolution of a particular behavior is not necessarily the reason for how it is maintained. For instance, kin selection is an important reason for the maintenance of cooperative breeding. It is becoming clear that cooperative breeding evolved out of monogamy. [4] But how does monogamy evolve? [5] We don't even know how an individual recognizes kin so how can an individual cooperate with individuals of other species?

Studies of mixed species flocks go back to 1863 but were sporadic until 1950, since then most of the studies happened. A recent meta-study has found 583 different studies about 682 different flocks from all continents and ecological regions except Antarctica. They define a mixed flock as such:

"A mixed flock is a roving group of three or more individuals from at least two species, moving in concert and behaving cohesively while foraging over a non-aggregated food resource." [6]

One of the objectives of the study is to create a glossary of terms to describe mixed foraging, the meaning of these terms is often confusing and confused in the literature. Here is a summary of the core definitions:

• nuclear - a species that contribute to the formation and/or cohesion of flocks
  

• leader - species at the head or forefront of a moving flock
  

• sentinel - species that broadcast alarm calls, evoking a clear fear response from other participants, which then either flush and/or remain silent for an extended period
  

• follower - species that trail after the leader when a mixed flock is on the move
  

Out of these simple definitions, the authors find 13 general categories of flocking behavior showing great diversity in species, territoriality, and seasonality. The reasons for multi-species flocking are increased food-capturing efficiency and protection from predators. Each of the four types of flock members might get different advantages so they must be looked at separately. Leaders, for instance, are more likely to be cooperative breeders, a measure of their sociability. [7] Mixed flocks can be quite stable, especially in stable environments like an untouched rainforest where the only changes are a tree falling occasionally. One study has found stable mixed foraging groups still together after twenty years. [8] This is several generations of birds, how this stability is maintained is unknown.

An experimental study of winter mixed-species flocks in Ohio in 1998 found that when leader / nuclear species were removed from isolated woodlots, the follower species had lower nutritional health based on new feather analysis than follower species where the leader / nuclear species were not removed. [9] The nuclear / follower species were Black-Capped and/or Carolina Chickadees and the Tufted Titmouse. The Chickadees are not cooperative breeders but highly social with strong male hierarchies and social monogamy. The Tufted Titmouse does have cooperative breeding with young staying with the nest for at least the next season. The species also have male hierarchies that extend into winter flocks. However the composition of winter flocks is not kin-based, young birds in a winter flock are as likely to be children of parent birds as not. It would seem that perhaps another dynamic is working [10] Why exactly followers gain an advantage being in flocks is not clear but predator protection is one possibility, also, bigger flocks move faster and cover more ground, thus finding more insects. In addition, within species aggression is higher than between species. Aggression within species makes sense in species with strict hierarchies like Chickadees and Tufted Titmouse but the lack of between species aggression runs counter to thinking of direct competition between species. [11]

Black-Capped Chickadee (Poecile Atricapillus) [12]

Carolina Chickadee (Poecile Carolinensis Atricapilloides) [13]

Tufted Titmouse (Baeolophus bicolor) [14]

Downy Woodpecker Female (Dryobates pubescens) [15]

Downy Woodpecker Male (Dryobates pubescens) [16]

White Breasted Nuthatch (Sitta carolinensis) [17]

Aggregation is when a group of biological individuals (cells, organisms, flocks, species, etc. interact to form behaviors that are termed "emergent," meaning that the results of the behavior somehow don't equal the sum of its parts. The study of statistical mechanics links the microscopic world of atoms with the macroscopic world of measurable matter. This often involves phase transitions, sudden changes of state, For instance, the freezing of water is a phase transition that only happens when water reaches a certain temperature. On the other hand, evaporation is a transition from liquid water to vapor that happens over a continuum of temperatures. Phase transitions are roughly analogous to behavioral emergence and because of this, there is hope that statistical mechanics can shed light upon biological aggregation. The analogy does have problems. Physics assumes the conservation of energy and equilibrium while biological entities may be close or even far from equilibrium. There is also a casual element. Atoms and molecules are acted upon only by outside forces, they are passive agents. Biological entities are computational, they process information from outside and act upon it. often change the state of the outside environment itself. They are active casual agents. Thus the physical study of biological aggregation is called active matter physics. [18] [19]

Starlings Flocking [20]

Starlings form cohesive and beautiful flocks of flying birds of anywhere from 200 to 50,00 individuals. These flocks can turn as one and if attacked by a predator can split apart and reform in a few seconds. In 2003 the University of Rome with a grant from the European Union started the STARFLAG project, a collaboration of physicists, biologists, and computer scientists to try to understand 3-dimensional flocking behavior better. [21] [22] The project has had a couple of breakthroughs. They were able to turn a stereo image into a 3D model and developed algorithms to track individual birds in the flock with 5% accuracy. In addition, they discovered two crucial rules about the perception and relationship of individual starlings in the flock. First, the starlings laterally arrange themselves, the arrangement is anisotropic. [23] This makes sense given the structure of bird eyes. The drawing below shows the three different types of avian eyes. Starlings are either (a) or (b), and predators like eagles are type (c).

Diagram of the different types of avian eyes [24]

Starlings adjust their movements within the flock by seeing the birds on either side of them. It was believed that the distance of these birds from the seeing bird made a difference but the second big finding of STARFLAG was that this is false, the seeing bird does not adjust its flight according to metric distance but according to topological distance, the number of specific birds that it sees. [25] [26] [27]

Relationship between a leader bird species (tufted titmouse) and a satellite species. [28]

Foraging is a special form of flocking as the birds are performing a cooperative, directed search for food. Whatever strategy is used it is assumed to have evolved as an optimal strategy. [29] Search can be modeled first as a random walk, Brownian motion. Actual data on search paths show them to be close to Lévy flights. [30] This is a random distribution that has a fractal structure, it is self-similar, zoom in to a graph of a Lévy flight and it looks the same. There has been some controversy as to whether this is the correct model for optimal foraging but most of the data seems to support it. [31] Most models assume no memory on the part of the animal. Another big question is whether this is an emergent property of the distribution of resources or whether it is evolutionary. Resource distribution can be modeled by a random point distribution called a Lévy dust, which is also self-similar. Campeau et. al., 2022 model the evolutionary fitness of Lévy flights versus a random walk. It showed some tradeoffs between the two types of search strategy but Lévy flights are better over the long run and for longer-lived animals. This confirmed Lévy flights foraging theory in an evolutionary context. The model used competition as a parameter but perhaps it can be expanded to cooperative foraging. The authors do suggest a possible connection to a game theoretic strategy called "hedge betting." [32] Lévy flights show up not only in human foraging but in human memory retrieval, we search our memories just like we forage, This suggests human memory is stored randomly. The suggestion is that Lévy flight scaling could be a universal scaling law in biology. [33] [34]

Lévy flight [35]

In 1963, in a paper dedicated to Konrad Lorenz on his 60th birthday, Niko Tinbergen laid out four questions that behavioral biology must answer for any behavior studied. [36] In the graphic below is a modern version of Tinbergen's four questions. Mechanism, Current Utility, Development, and Evolution. It is useful to look at mixed foraging flocks using these four questions. I would say that very little is known. the mechanism is perhaps audio memory updated daily in the dawn chorus. Mixed species foraging seems to improve foraging quality for all members but it has been hard to tease out individual fitness costs. I couldn't find out much about development. There has been mention of the coevolution of large flocks in the tropics but I found no real studies. Why are some tropical flocks color-coordinated? Nothing about smaller temperate flocks. To my knowledge, the link between leader species and cooperative breeding has not been explored.

Tinbergen’s Four Questions [37]

The four questions of Tinbergen are interconnected inquiries, one can provide data for the other. Behavior is about interaction and relationships, and the basic structure of relationships is the network. Social networks can show structure between species, within species, and between all individuals in a mixed flock. Getting the data itself is difficult yet new methods, some noninvasive, are being developed. Networks are inherently non-linear, statistics for networks consist of randomization tests, general linear models, and Bayesian inference. Network structure forms a baseline and interface between raw data and the biological questions being asked. [38] [39] [40]

I never knew until researching the Dawn Chorus that mixed-species flocks existed. Behaviors like this, which are often overlooked by the standard narrative are biologically important and enrich us all.

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15. Thomas, Ken. A Female Downy Woodpecker (Picoides Pubescens). Photo Taken with a Panasonic Lumix DMC-FZ50 in Johnston County, North Carolina, USA. January 8, 2007. KenThomas.us(personal website of the photographer). https://commons.wikimedia.org/wiki/File:Downy_Woodpecker-Female.jpg.
    

16. Thomas, Ken. A Male Downy Woodpecker (Picoides Pubescens). Photo Taken with a Panasonic Lumix DMC-FZ50 in Johnston County, North Carolina, USA. January 14, 2007. KenThomas.us(personal website of the photographer). https://commons.wikimedia.org/wiki/File:Downy_Woodpecker-Male.jpg.
    

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