Ornithology Lecture 2

1. Single basioccipital condyle

2. Lower jaw of several bones & movable quadrate

3. Intertarsal ankle joint

4. Pneumatic bones

5. Scales

6. One earbone in middle ear (not 2 like mammals)

7. Females are heterogametic sex (zw), some reps also

8. Blood proteins

9. Oviparity

10. Nucleated red blood cells

1. Homeothermism

2. Four-chambered heart with double circulation

3. Feathers

Archaeopteryx (approx. 140 mya)

Our scenario about the evolution of birds from reptilian ancestors was stymied for quite some time by the absence of adequate fossil material to support the hypothesis. This problem was rectified in 1861 with the discovery of the first fossil specimen of Archaeopteryx lithographica in Bavaria. Since that initial discovery, 3 additional fossil specimens have been unearthed in the same region. The most recent "discovery" was actually recovered from the private fossil collection of an amateur fossil collector in Solnhofen, West Germany.

Significance of Archeopteryx

Archeopteryx has been called the "most important natural history specimen in existence". There are 3 reasons why Archeopteryx is considered so important.

1. Completeness of the specimen

2. Intermediate nature of many of the characters- this explained one of the most glaring holes in the fossil record- how birds evolved from reptiles

3. Timing- The first Archeopteryx was found in 1861 just 2 years after the publication of Origin of species by Darwin in 1859.

Characteristics of Archeopteryx

Avian Characteristics	Reptilian Characteristics
1. Feathers	1. Scales
2. Large furcula	2. Long bony tail
3. Well developed coracoid	3. Clawed fingers
4. Hallux back-facing	4. Abdominal ribs
5. Pelvis intermediate	5. No keel
	6. Teeth
	7. Single occipital condyle
	8. Skull structure
	9. Metacarpals unfused

There is considerable debate over the specific reptilian origins of birds. Both of the current hypotheses suggest that birds originated from a line of mesozoic reptiles. One theory suggests that birds originated from an older group of thecodont reptiles, the other suggests a more recent origin from theropod dinosaurs.

1) Thecodont or Crocodilian Sister Group Theory -Originally proposed in 1888 and elaborated on by Heilman in 1927. It is now favored by several modern paleontologists. The theory proposes that birds evolved from lightly built reptiles that gave rise to many other groups including crocodiles. Some thecodonts were very similar to birds and were arboreal.

- Avian lineage separates by mid-Triassic (200 mya) or earlier ANCIENT ORIGIN

- Ancestor was quadriped

- Consistent with arboreal origins of flight

- Endothermy evolves as a result of flight as would feathers

- Supported by similarities in skull and dental characters, - 14 skeletal characters are shared by birds and crocodiles

Problems

- difficult to explain long gap in fossil record

2) Theropod or Pseudosuchian Model This theory was first proposed by Huxley in 1868. The theory proposes that birds are derived from small dinosaurs that were contemporaneous with Archeopteryx. Compsognathus, a small bipedal fast running dinosaur, was identified as a potential ancestor.

- Proavis seen as an obligate biped

- Consistent with cursorial origin of avian flight

- Endothermy and feathers would have evolved in a pre-flight running phase

- Share 23 of 42 specialized skeletal features in the hand, vertebrae, humerus, ulna, hindlimb, and pelvis disputed

Problems

- Difficult to explain evolution of flight in cursorial animal

Based on the similarity between feathers and scales, etc. it is commonly thought that birds evolved from reptiles. With regard to flight capabilities, the question remains - how did the forelimb of reptiles evolve into the flight structure of birds?

There are 2 general evolutionary hypotheses regarding this question. Wings and flight may have evolved in one of the following ways:

Terrestrial

1) Running/terrestrial reptiles may have flapped their forelimbs to aid in locomotion. Subtle changes in the morphology of scales gave rise to feathers and increased efficiency at locomotion. Natural selection did the rest.

2) A variation on the terrestrial argument suggests that bird wings evolved as food gathering structures and later evolved into flight adaptations. The hypothesis states that insectivorous birds used wings to corral insects.

Arboreal

3) Finally, an arboreal hypothesis suggests that flight capabilities evolved in tree-dwelling reptiles that clambered around in the canopy or subcanopy. These reptiles possessed claws (like the Hoatzin) that they used to hold on to the limbs. These reptiles glided first and later evolved flapping flight.

These are interesting ideas about the evolutionary origins of flight in birds. But they cannot be evaluated. It is difficult to go back and test such evolutionary hypotheses critically. All one can do is to amass corroborative data to support one or another argument.

Fossils Since Archaeopteryx

The fossil record of birds is not great, owing in part to the delicate nature of their skeletal features. Their bones have fossilized poorly, thus we do not have a good record of their history. What we do have is somewhat patchy. We will work through the fossil history of birds starting at the time of their origin in the Jurassic and moving forward to Recent times.

Cretaceous: (140-65 million yrs ago)

- The physical appearance of birds essentially came to resemble modern birds a short 30 million years after Archaeopteryx

- Some species possessed a keeled sternum (e.g. Ichthyornis small bird that resembled a gull), but others did not have carina (e.g. Hesperornis- loon like aquatic bird)

- Similarly, some species had teeth and others did not; toothed birds became extinct at end of cretaceous

Fossils- Charidriformes, Procellariformes

Paleocene: (65-53 mya)

Two major groups show up in fossil record, Coraciformes, and Strigiformes

One unusual N. American species that existed was Diatryma, a large predaceous land bird that stood 2 meters high and had a fearsome hooked bill. In N. America, Diatryma disappeared in the Oligocene at about the time that placental mammals appeared.

Eocene: (53-37 million yrs ago) Green River, WY deposits

- A period of major bird evolution; 80% of modern avian orders came into existence.

Oligocene: (37-26 million yrs)

- Major period of radiation in fossil record. Most modern orders of birds were present by the end. Many modern families and some genera present (Aquilia - Eagles, Phoenicopterus -Flamingos, Charadrius -Plovers and others.

Groups seen in fossils: Cuculiformes, passeriformes, Anseriformes, and Peleconidae

Miocene: (26-7 million yrs ago)

- Most modern families and genera were present. Still, some modern families first showed at this time (e.g. falcons, crows, thrushes, and wood warblers).

Pliocene: (7-2 million yrs)

- All modern bird genera were established by this time.

Pleistocene: (2 million yrs- 10,000 yrs)

- A period of dramatic climatic change that was associated with great shifts in the biotas. The series of glacial and interglacial periods caused tremendous shifts in the distribution of species. Moreover, many species went extinct owing to climate change. Estimates are that between 24-32,000 species evolved during the Tertiary period, thus the Pleistocene can be viewed as a period of major extinction, yielding the current 9000 species present today. - Evolutionary change during the Pleistocene was probably limited to the development of geographic subspecies or races of birds owing to bouts of isolation by glaciers.

- More recent, historical data indicates that many flightless species existed during the Pleistocene. In New Zealand, for instance, there were 14-20 species of Moas split into two families (Dinornithidae & Anomalopterygidae). In Madagascar, the Elephant birds (Aepyornithiformes) roamed. These birds were probably exterminated by the expanding cultures of indigenous humans.

Recent: (---> 11,000 yrs)

- The past few thousand years represent a minor window in the sequence of avian evolution. What changes occurred were most likely minor adaptations to local environments. The dominant taxa (in terms of # of species) became the Passeriformes. Continuing glacial epochs probably caused repeated episodes of extinction. Obviously, however, human influence accelerated extinction of many taxa (e.g. Moas, Dodo, Great Auk, Passenger Pigeon, Carolina Parakeet, etc.)

What is a species?

1. Biological species concept- Ernst Mayr 1942

"a species is a group of actually or potentially interbreeding natural populations which are reproductively isolated from other groups"

Strengths- Species defined by potential for gene flow between populations rather than by arbitrary morphological criterion.

Weakness- Difficult to determine reproductive isolation of allopatric populations. Populations that do interbreed may still be evolving independently of one another. May lose information about the evolutionary process by lumping groups.

2. Phylogenetic species concept- Mckitrick and Zink 1988

"smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent"

Strengths- Species defined by specific morphological or genetic characters. Preserves all of the evolutionary information in the lineage. Uses basic unit of evolution.

Weaknesses- Neglect of information on hybridization and assortative mating. May use trivial or unimportant characters. Produces unwieldy classification.

Speciation Models:

1) Allopatric- geographic isolation followed by divergence

2) Parapatric- strong selection across a species' range causes divergence in absence of isolation

3) Sympatric- speciation occurs in the same location without any spatial separation

According to Ernst Mayr, the most prevalent speciation mechanism (allopatric) traces the following sequence:

1) A formerly contiguous population becomes geographically isolated

2) Small evolutionary changes occur in the populations owing to natural selection, mutation, genetic drift, etc.

3a) The populations diverge sufficiently to become new species with no gene flow between them

3b) The two populations come into secondary contact. There are two possible outcomes.

1. If the populations had diverged sufficiently, there will be selection for reproductive isolating mechanisms that will reduce gene flow between the populations

2. If there was insufficient divergence, the two populations will coalesce into one species again

Premating Mechanisms:

1. seasonal and habitat isolation

2. ethological isolation (Meadowlarks)

3. mechanical isolation

Postmating Mechanisms:

1. gametic mortality

2. zygote mortality

3. hybrid sterility

4. hybrid inviability

examples of the evolution of isolating mechanisms

Western and Clark's Grebes assortative mating based on call notes

Eastern & Western Meadowlark hybrids were either sterile or they had low hatchability and low fledging success

examples of apparent coalescence of populations

Yellow-rumped Warblers and Northern Flickers

Adaptive radiation- Rapid evolution of a diversity of ecological types from a common ancestor.

examples) Darwin's Finches, Hawaiian honeycreepers

Evolution- Directional change in gene frequencies over time.

In order for evolution to occur there must be genetic variation within the population. Two forces may cause gene frequencies to change in a directional fashion over time; genetic drift and natural selection. Before we discuss these topics lets first look at how genetic differences can be expressed.

Patterns of variation

We see evidence of divergence of populations within many bird populations. For example, many species are divided into distinct subspecies or races based on morphological differences. One third of the species in NA are divided into distinct subspecies. If these differences become large enough, this may lead to speciation.

OVERHEAD

ex) Variation in Fox Sparrows in the western US

These patterns of variation often are consistent from one species to another. Two patterns that have often been observed in homeotherms such as birds are Allen's rule and Bergman's rule.

Allen's rule- birds in colder climates generally have shorter beaks, wings, and tarsi than those populations in warmer climates.

Bergman's rule- populations in colder climates (higher latitudes) have larger bodies than population in warmer climates (lower latitudes). This is true in about 90% of the races of resident Palearctic species.

Gloger's rule- populations in warmer and more humid climates have darker coloration than those in cooler or drier climates.

These kinds of changes can occur rapidly.

ex) House Sparrows in New Zealand- within 100 years of their introduction, House Sparrows living in different regions have evolved differences in size and coloration consistent with the Allen's, Bergman's, and Gloger's rule.

Background: Sibley, C.G. et al. 1988. A classification of living birds of the world based on DNA-DNA hybridization studies. Auk 105:409-423.

Classification: Orderly arrangement of species.

Systematics: Science of classifying organisms based on evolutionary relationships. The first classification of birds was published in 1676. Birds were divided into two main groups, land birds and water birds.

Homology: Similarity in structure due to common origin.

ex) penguin flipper, and wing of a petrel.

Convergence: Similarity in structure, different origin (sometimes called analogous characters).

ex) wing of insect and wing of bird.

ex) Bill morphology of sunbirds and hummingbirds.

ex) Foot morphology of hawks and owls

ex) zygodactyl foot pattern evolved 10 times in different groups

Challenge: To distinguish homologous characters from convergent or analogous characters. Must consider characters that are conservative and not liable to rapid change by natural selection.

Taxanomic characters must be homologous structures that "can be traced phylogenetically to the same feature in the immediate common ancestor of both organisms." The primitive character state is the from which the derived state evolved. It is assumed that 2 species with the same derived character state have a common ancestor with the same character state. It is best if both the primitive and derived states are available for examination.

ex) Penguins flippers- flippers of all penguins are presumably derived from a common ancestor. The primitive state can be seen in petrels, the closest living relative of penguins.

Cladogram- Diagram of hypothetical phylogenetic relationships. The point at which character states change are used to define relationships. Assume that the cladogram with the fewest evolutionary changes is the most plausible hypothesis. Best to use many characters.

1) Morphology

a) Size (song sparrow, fox sparrow geographic variation is considerable)

b) Skeletal features (carinate vs. ratite birds; palate structure; # and shape of cervical vertebrae)

c) Musculature (leg muscles; jaw muscles; tongue muscles in parrots; syringial muscles of songbirds)

d) Anatomy (vas deferens opening; intestinal coiling)

e) Plumage (feather distribution and molting patterns in shorebirds; waxes associated with uropygial gland of waterfowl)

The problem with morphological characters is that they may be strongly influenced by environment resulting in evolutionary convergence.

ex) Unrelated nectar feeding birds around the world have long thin bills and bright plumage because of similar selection pressures.

2) Behavior- Courtship displays (avocets, lovebirds); maintenance behavior (direct vs indirect scratching); songs (empidonax flycatchers, white-crowned sparrows). Generally used to separate closely related species, not reliable for higher taxonomic classification.

3) Parasites- Feather parasites are highly "coevolved" with many bird species. Thus they are fairly dependable in recognizing evolutionary relationships (cuckoos have their own parasites not their hosts)

4) Geography- Species resembling each other generally exhibit geographical proximity, but this is a problematic cue based on the volant abilities of birds and high dispersal capabilities.

5) Chromosomes- Not very successful with birds because there is little variation across species and few species have been karyotyped (<5% of the 9000 species have been examined). Analyses to date suggest that chromosome changes in birds do not accompany speciation.

6) Biochemical Techniques

a) Electrophoresis- Uses tissues (blood, feather pulp, muscle of heart or breast, liver, etc.). Examines allelic variation within and between populations. Less chance of convergence in protein sequences, hence this is more reliable than morphological criterion.

Birds exhibit little differentiation between species and subspecies compared to other vertebrates making this technique less powerful for birds. Perhaps this is related to dispersal capabilities or high body temperature.

b) DNA-DNA Hybridization- Uses strands of DNA and gauges evolutionary relationships based on how "tightly" 2 species DNA combine following denaturing and heating.

Technique involves: 1) isolate DNA from 2 species; 2) label radioactive strand; 3) mix species DNA so that they reanneal along homologous DNA sections; 4) Heat the mixture and record the proportion of DNA released at temperature increments.

The phylogenetic distance between 2 species is measured based on the 50% dissociation temperature. Higher temperatures required to dissociate strands = greater DNA similarities and, therefore, closer evolutionary ties.

Findings of Sibley et al. (1988):

a) Ratites group together

b) Owls with goatsuckers

c) New world vultures with storks

d) Wrentit is a babbler

e) Passeriformes restructured

f) Major diversification of birds in Australian region

There is considerable criticism of the DNA-DNA technique, especially as practiced by Sibley et al. Specifically, no one has seen the data and 50% dissociation temperatures are criticized.

c) mtDNA Analysis- Mitochondrial DNA evolves more rapidly than nuclear DNA and is maternally inherited. It can provide "high resolution" information on the evolutionary relationships of species.

Combination of survival and fecundity patterns that are characteristic of a species or population.

General life history patterns of birds

1. Low 1st year survival, constant from 2nd yr on

2. Reproductive success improves with age

variation

3. Species either have

a. low surv. and high fecundity r sel

ex) passerines, ducks, small shorebirds

or

b. high surv. and low fecundity K sel

ex) Large hawks, owls, albatrosses

4. Ecology molds life history in predictable ways

a. clutch size increases in N

b. survival higher in S latitudes

An interesting question in the evolution of life history characteristics is: What is the tradeoff between survival and reproduction?

Presumably, maximization of these parameters leads to the maximization of fitness.

Studies of some birds have shown that there are tradeoffs between reproduction and survival. When reproductive effort is increased (usually by increasing CS) this leads to a reduction in survival or future reproduction in these individuals.

This is not always the case, however.

The pattern of survival varies greatly from species to species but there are 2 general patterns that are seen in most birds.

1. Young survive more poorly than adults

2. Once birds reach adulthood (1 yr) survival is constant

OVERHEAD

ex) Herring Gull- 60% surv 1st yr, 90% thereafter

ex) Great Tits- 20% , 48% thereafter

ex) Royal Albatross, Bald Eagle, Atlantic Puffin- adult surv 95%

The pattern of fecundity also varies greatly from species to species. There are a number of ways that fecundity can vary and a number of ways it can be measured.

Measured

1. Yearly- number of young successfully fledged/pair

2. Lifetime- number of young fledged by one indiv. in their lifetime- rarely measured but very important when considering evolution of traits because this is what selection is acting on.

Sources of variation

1. Clutch size- no. eggs in clutch

2. Number of clutches per year

3. Nesting success

4. Age at first reproduction

5. Interval between breeding

Lets look at some of these

Age at first reproduction

Most birds breed in their first year but some delay breeding for several years.

Age Species

1 most passerines

2 Swifts, some passerines

2-3 Parrots

3+ Raptors

4 Large waterbirds, Seabirds (gulls, murres, etc.)

8-12 Large albatrosses and Condors

There is a great advantage to breeding early in life in terms of the potential to increase lifetime fitness.

Why do some species delay breeding?

Adelie Penguin

1. Not reproductively mature

2. 2-3 years of experience are needed to learn how to forage efficiently

3. 1 yr of social experience is needed to develop behavioral skills for successful pairing and nest defense

Variation in nesting success

1. Nesting success varies as a function of the type of nest.

hole nesters > cup nesters > ground nesters

time involved in excavating and defending cavity nest pays off in terms of increased nesting success

Primary cavity nesters- excavate their own nest hole- woodpeckers

Secondary cavity nesters- use holes excavated by other species or natural cavities

There generally is intense competition between secondary cavity nesters for access to nest sites.

2. Nesting success varies as a function of lattitude.

temperate zone species > tropical species

This may partly explain why species migrate north to breed.

Variation in number of broods

The number of nesting attempts that a bird makes in a season varies both within and between species.

Refers to separate successful nesting cycles in a season.

Replacement clutches- Refers to renesting after failure. Even birds that nest only once in a season may produce replacement clutches if the failure occurs early.

The number of broods attempted generally increases across species from N to S. This is a function of the length of the breeding season.

ex) Song Thrush- 1 attempt in N 65-70 degrees N

2-4 attempts in S 45-50 degrees

ex) Doves and Pigeons- 2 eggs max, many attempts

ex) Chickadees- up to 12 eggs, generally 1 attempt

Evolution of Clutch Size

Clutch size or the number of eggs a female lays varies greatly within and between species.

ex) most Passeriformes have clutches of 2-12 eggs

ex) Anseriformes, Galliformes, Gruiformes - usually have large clutch sizes of 6-20 eggs

ex) Shorebirds (especially Scolopacidae)- almost always have a clutch of 4

ex) Hummingbirds and Columbiformes have clutches of 2 eggs

One of the central questions concerning life history patterns in birds is what determines clutch size?

There are 4 hypotheses that are currently being debated.

1) Lack- parental ability to care for young limits clutch size.

2) Tradeoff- Clutch size represents a balance between increased reproductive effort and future survival.

3) Predation- Predation selects for small clutch size because more young are more conspicuous to predators.

4) Seasonality- Clutch size reflects the season availability of resources relative to population size.

I will discuss two of these hypotheses in more detail but you are responsible for understanding all four of them. There is support for and problems with each of these hypotheses. Please read the section in the text book pertaining to this material (pp. 418-424).

Lack's hypothesis- Lack's hypothesis predicts that the most common clutch size in the population will also be the most productive in terms of the number of young that are successfully recruited into the population.

This prediction is generally supported but there are exceptions. In some cases it has been found that the most productive clutch size is larger than the mean clutch size in the population (see fig. 21-7).

In addition, Lack predicted that clutch size increases in northern populations because the parents have more time to gather food and feed the young with longer days. However, owls and other nocturnal birds have larger clutches in northern locations and therefore the day length idea is inconsistent with the pattern in these species. Also, Lack's hypothesis does not explain the increase in clutch size with increasing altitude.

Seasonality hypothesis- The seasonality hypothesis predicts that clutch size is a function of the difference in productivity between the summer and the winter. In highly seasonal environments, there will be more "surplus" food available for birds to put into breeding. Thus, variation in per capita food availability causes differences in clutch size. This hypothesis currently has the most support.

ex) Clutch size variation in Northern Flickers is consistent with this hypothesis.

Community Ecology- the study of natural communities. Ornithologists have played a large in the development of the modern ideas in community ecology.

A biological community can be described in a variety of different ways. In most cases a community is defined as a group of coexisting species that interact with one another.

Ways of Defining Communities

You undoubtedly have encountered many ways of defining or identifying communities.

1) Habitat types - rocky intertidal, tundra, savannah...

2) Life forms - chaparral, old growth forest...

3) Taxonomy - bird, lizard, fish communities...

4) Resources - nectar feeders, seed eaters

- may include species from many groups

Defining communities by taxonomy can be arbitrary, however, and may ignore important interactions.

Example) Studies of the impact of insectivorous birds on invertebrate prey populations have ignored the effect of other arthropods, small mammals and reptiles as potential predators and competitors.

The central questions in community ecology are:

1) what determines the number of species that occur in a particular area and

2) what is responsible for changes in species diversity from one location to another.

The concept of species diversity is central to many of the questions in community ecology. Species diversity may be defined in a variety of ways but there are 2 definitions that are commonly used.

1. Species richness- the number of species in a community.

ex) If the community is the birds of Arcata Forest, species richness would be the number of species that occur in the forest.

2. Species diversity- a measure of the number and relative abundance of species in a community. This is usually defined as

H'=-p_ilnp_i. Where p_i=the proportion of the ith species.

Patterns of species diversity

The number of bird species that breed in an area increases as you move towards the tropics. For example, 56 breeding species are found in Iceland, while there are 135 in New York State, 550 in Honduras, and 1300 in Columbia. Within Columbia you also find high variation. 47 species breed near tree line in Paramo vegetation while approximately 1000 breed in lowland rain forest.

What causes these differences?

1. Foliage height diversity- One idea put forth Robert MacArthur was foliage height diversity. FHD is a measure of the distribution of the vegetation among height classes. Thus grasslands have very low FHD shrublands are sightly higher while multi canopied tropical forests have the highest. While FHD explained much of the variation, other factors are certainly involved.

2. Novel resources- Another factor that explains much of the increase in bird species diversity in tropical regions is the addition of new resources. There are a number of qualitatively different resources in tropical forests that do not occur in temperate forests.

a. Large fleshy fruits- supports large frugivore guild in tropics (Parrots, Toucans, Trogons, etc.)

b. Large insects- many bird species specialize on this resource

c. Dead leaves- a predictable resource used by several tropical bird species

d. Epiphytes- flowers and insect resources that require special adaptations

e. Ant followers- whole families of birds (antbirds) that obtain most of their food by following army ant swarms and feeding on the insects that are flushed by the ants.

3. Reduced Niche Breadth- narrower niches of tropical birds allow more species to be packed along one resource dimension.

4. Habitat heterogeneity- greater in tropical habitats

5. Time- more time for speciation and evolution of specialized niches in the tropics. Example of neotropical versus African forests and grasslands.

ex) Old world vs New world tropics

Rainforest- Neotropics > Africa

Grasslands- Africa > Neotropics

Rainforest vegetation is older in the neotropics and grasslands are older in Africa.

The study of avian communication has been prolific, in part, perhaps because birds use the same general communication channels as humans (visual & acoustical). Also, since song is learned in many bird species, it provides a model for evolution of cultural traits.

Definition: Communication occurs whenever the actions of one animal influence those of another.

Vocal Communication

Birds vocalizations are of two general types:

1) Calls

The distinction between these two is generally based on complexity. Calls are brief sounds with relatively simple acoustic structure, usually mono- or dysyllabic and usually don't involve more than 4 or 5 notes. Calls may occur in bursts of extended duration (alarm calls, flocking calls of Chickadees), but there is no clear organization or pattern detectable.

2) Songs

A song is more complex than a call and "consists of a group of notes separated from another group by a pause longer than the pauses between notes themselves".

Nonvocal, Sound Communication (not covered in lecture)

Many birds communicate via acoustical cues that are not vocal in origin, but that are produced by other structures of the bird.

1) Beaks - Storks, owls, etc. clap their beaks during courtship displays.

2) Feathers - Ruffed Grouse, Common Snipe, and Common Nighthawk produce sounds using feathers.

3) Esophageal Pouches - Several grouse species emit sounds by inflating and deflating air sacs.

4) Feet - Sharp-tailed Grouse stomp their feet on the display grounds, producing a rapid, drumming sound.

Visual Communication (not covered in lecture)

Birds make use of other media to communicate, most notably visual cues. The elaborate plumage and behavior of males of many species (e.g. Birds of Paradise) are hypothesized to have arisen via sexual selection. Several species even make use of their environment to attract mates (e.g. Bowerbirds).

1) Plumage coloration and behavior - Birds of Paradise combine song displays with elaborate visual displays.

2) Flight Patterns - male Wilson's Phalaropes communicate with females other than their mates by wing-fluttering. This behavior communicates the male's unavailability as a potential mate. Male hummingbirds also make use of aerial displays to attract mates.

For the remainder of the lectures on communication, I want to focus on song, since it is the most obvious and well-studied component of communication in birds.

There are three general areas used to describe the functions of song. These areas are general answers to the question of why do birds sing. These are answers to evolutionary questions about why individual birds sing.

1) Reproduction

In most bird species, especially the songbirds (Passeriformes: oscines), it is usually the male who sings. In relation to reproduction, two general reasons can be given for why individuals of one sex sing:

a) Repel rivals - research has shown that males sing most vigorously during the peak of the breeding season. One explanation for this is that males (of territorial species) are defending territories.

One way of testing whether song functions to repel rivals or defend territories is to remove males and replace them with loudspeakers that broadcast songs. The results of these experiments generally have shown that song (in the absence of a male) will repel rivals for short periods of time. Moreover, more complex song (multiple repertoires) repels rivals for longer periods of time.

b) Attract mates - certainly song attracts opposite sex partners as mates. Again, experimental evidence using songs that vary in complexity has shown that males that sing more complex songs have higher mating success.

Song may also have proximate influences on the physiology of individuals in relation to reproduction. It may: 1) strengthen the pair bond; 2) stimulate & synchronize breeding between mates; 3) identify individuals, either to a mate, parent or offspring; or 4) signal change in behavior such as parental duties.

2) Survival

Most of the social functions of avian vocalizations fall into the domain of calls. Thus, simple vocalizations may aid in the survival of individuals by enhancing

a) Antipredator responses

1. Predator mimick- hisses of Burrowing owl sound like rattle snake

2. Alarm call- faint, thin, high pitched calls that conceal the position of the sender.

b) Communication beytween group members- flock cohesion during migration (geese, cranes, songbirds vocalize during migration) or other movements (mixed species flocks foraging in densely vegetated areas.

1. Localizable call- short notes with broad frequency range that give information on the location of the sender.

Call types

1. Localizable calls- short notes with broad frequency range

3) Orientation

Some species of birds (Oilbird, Steatornis; Collacallia swiftlets) vocalize to provide acoustical cues used in echolocation. Both of these species inhabit caves and use high frequency sounds much the way bats do.

Ecology of Bird Song

Based on what you have observed during the field labs in this class you probably could make some argument for a relationship between types of bird songs/calls and habitat. The foundation of this argument is that natural selection has shaped the optimal vocalization in a particular habitat. For instance, individuals that sing or call using vocalizations of a particular frequency may be more successful at communicating to conspecifics.

What evidence is there that habitat has influenced bird communication?

Interspecific Comparison

Morton (1970) analyzed the physical features of bird in forest and grassland habitats and found the following differences.

Emphasized Frequency % Pure Tones Frequency Range

Forest 2.2 Khz 87 1.5

Grassland 4.4 Khz 33 3.5

What does this mean?

In terms of sound images, forest birds have songs with many low-pitched, pure whistles (Bellbird, Hermit Thrush, Varied Thrush), whereas grassland species tend to have more high-pitched, buzzy trills (Clay-colored Sparrow, Marsh Wren).

Experiments showed that songs recorded and played back in native habitats retained more of the original physical characteristics than foreign songs. Songs attenuated less in native habitat.

Intraspecific Comparison

Further evidence that song variation correlates with habitat comes from within species comparisons of populations in different habitats.

1) Rufous-collared Sparrow: Grassland birds have faster trill rates than forest birds.

2) Great Tit: Songs of birds in an English park are more similar to songs from birds in similar habitat in Iran than they are to English birds in dense forest less than 100 Km away.

Song Development

One of the most intriguing questions of animal behavior concerns the extent to which behavior is learned vs. innate (the nature/nurture argument). The study of song development in birds has provided valuable information offering new insights into this question.

Early Evidence

Early experiments in the study of bird song were of two kinds:

1) "Kaspar Hauser" experiments in which young birds were raised in total isolation, thus preventing them from hearing songs of conspecifics or other species.

Results: Songs of isolated birds were a) slower, b) more variable, c) tended to have fewer elements, and d) had other abnormalities.

2) Birds deafened (severing auditory nerves) @ approximately 5 months age.

Results: Abnormal song.

Recent Studies

More recently, Peter Marler et al. have developed a model for song development in birds based on their work on White-crowned Sparrows (Zonotrichia leucophrys) and other songbird species.

The following model derives from observational studies of song development in WCSP:

Normally,

Hatching->10-50 days ----->150 days------>200 days

Critical Period subsong fullsong

Summary: (1) Hatching occurs in the spring and early summer. Birds are born with a crude (genetic) template, which constrains what is learned. This template varies tremendously among species and is correlated with variation in song learning (contrast the elaborate mimicry of Northern Mockingbirds, Mynas and Parrots with the limited vocal repertoire of many other species).

(2) Approximately 10-50 days later a critical (sensitive) period occurs in the development of males during which they hear song of other males in the population. This is referred to as the memorization (or perceptual learning) phase, and it is strongly influenced by hormones, daylength and social factors.

(3) About 150 days after hatching juvenile males begin the motor (or motor learning) phase, which involves singing subsong (a muted, incomplete version of full song). At about 200 days adult males first begin full song.

The model can be summarized as:

Mature Male Song + Critical Period + "auto-training" = Full Song

Variations

What we know of song development in birds is:

Song is innate (exclusively determined by genetics) in some taxa (e.g. Galliformes, Columbiformes, and some suboscines i.e. tyrannidae) and learning does not occur.

In other groups, the genetic template for singing a particular song is obviously present, but vocal learning occurs in these groups (e.g. most Passeriformes, Apodiformes, Psittaciformes).

For the songbirds (oscines), considerable interspecific variation exists. This variation in the critical period occurs in the following fashion:

1) Learning is restricted to a brief sensitive period (WCSP).

2) Learning occurs as a young adult.

3) Learning occurs during first 1.5 years

4) Learning occurs throughout life (mockingbirds, mynas, starlings, parrot = excellent mimics)

Song Dialects

One of the most fascinating facets of avian song involves the interaction between song learning, ecology, and evolution. In many species, but especially in the White-crowned Sparrow (Zonotrichia leucophrys), it has been recognized that individuals exhibit subtle geographic variation or mosaics (dialects) in their songs. In many species, songs of neighbors are more similar to each other than are those of more distant birds. In other words, much as humans have dialects across our worldwide/local distribution - so too do some birds.

Example) WCSP at Golden Gate Park

This has been best studied in the local populations of WCSP that inhabit the vicinity of Golden Gate Park. Here, Baptista et al. have identified a number of distinct dialects that coincide with local geography. Remember: 1) males have an early sensitive period during which song develops; and 2) males tend to breed very near where they are hatched.

The result is a mosaic of dialects with quite sharp boundaries.

Functional Significance of Song Variation

Given these patterns of geographical song variation (dialects), one may be lead to ask why is it that such variation has arisen? This is an evolutionary question about why it benefits individuals to sing a particular dialect? or what is the evolutionary significance of the dialects?

Two possible explanations have been suggested to account for the presence of dialects.

1) No Functional Significance: In effect, this is a null model stating that geographic variation occurs as a byproduct of vocal learning and it has no evolutionary significance.

The problem with this explanation is that it doesn't hold for species with sharp dialect boundaries unless these boundaries correspond to geographical barriers to dispersal.

2) Mate Choice: An alternative explanation argues that natural selection favors song learning because it gives rise to variations that are advantageous. Why are they advantageous? Because it permits individuals (females) to mate assortatively. What this means is females mate with males of "their kind". The argument here is that dialect is used as a cue by females to choose males that were hatched locally. Presumably those females that mate with males of the local dialect derive genetic benefits. Mating with other individuals of the same area may preserve coadapted gene complexes making their offspring better adapted to the local environment.

The problem is there is no evidence of genetic differentiation across dialect boundaries suggesting there is no barrier to movement of individuals across these boundaries. Also, young colorbanded in one dialect have been observed to move across dialect boundaries and sing the song of their "adopted" dialect.

3) Male-male interactions- It has been observed that young males who mimic the songs of their neighbors suffer less aggression than males that sing different songs. Thus, there is strong selection to "conform" to the native dialect. The reason for the reduction in aggression is not clear but may derive from other males being fooled into thinking the newcomer is a dominant established male.