Birds

What makes a bird a bird?

•         Feathers!

•         Birds are evolved for flight.

▫         This somewhat limits their diversity relative to other groups of animals.

 

Physical Adaptations of birds: most are related to flight

•         Feathers: lightweight but remarkably tough and strong

▫         Vaned:

▫         Down:

▫         Contour:

–  Contour Feather Anatomy

•         Hollow quill: emerges from skin follicle

•         Shaft: continuation of quill

•         Barbs:

•         Resemble miniature feathers with barbules spreading out laterally from barbs

▫         Feather growth

–  Feathers first develop from epidermal elevation of skin.

•         Do not flatten like scales in reptiles, but instead roll into a cylinder and sink back into the follicle

–  Feather then continues to grow.

–  Near end of growth, keratin is deposited.

–  Protective sheath splits apart, allowing end of feather to protrude and barbs to unfold.

▫         Feather molting

–  Highly ordered process:

–  Flight and tail feathers are lost in exact pairs.

–  Replacements emerge before the next pair is lost.

–  Many water birds lose all primary feathers at once and are grounded during molt.

–  Penguins molt all feathers at once.

•         Skeleton: key features

▫         Skeleton must be light but sturdy.

–  Bones are pneumatized:

 

–  Vertebral column is rigid:

 

–  Ribs are mostly fused with vertebrae, pectoral girdle, and sternum for increased stability.

▫         Sternum

–  Often with large, thin keel providing an attachment site for flight muscles

▫         Pectoral and pelvic appendages

–  Bones of forelimbs highly modified for flight

–  Bones of hindlimbs somewhat modified as well for walking.

▫         Skull

–  Light, but with large braincase and eye orbits

–  Teeth missing

•         Muscles

▫         Large pectoralis muscle:

▫         Supracoracoideus muscle:

▫         Both muscles anchored to keel and low on body:

▫         Main leg muscles in thigh around femur

▫         Smaller mass lies over tibiotarsus (drumstick)

▫         Feet nearly devoid of muscles

–  Covered by tough scaly skin

–  Toe-locking mechanism

•         Endothermy

▫         Increased locomotion requires increased metabolic rates. Birds have high metabolic rates=lots of heat produced during their skeletal activities.

▫         This heat can be maintained in the body due to the insulative properties of feathers, thus enabling them to be endothermic.

▫         Benefits:

–  More flexibility in habitat and activity patterns

▫         Problem:

–  Requires large amounts of food to be brought in

•         Digestive System

▫         Efficient digestive equipment is necessary to process as much energy as possible from food.

▫         Components include:

–  Beak:

–  Gizzard:

–  Salivary glands: poorly developed but provide mucus for food passage

–  Tongue:

–  Pharynx leads to esophagus to stomach

–  Crop:

–  Stomach leads to intestine and rectum, ending in cloaca

–  Cloaca is also the common receiving area of genital ducts and ureters.

▫         Water conservation

–  Kidneys: form highly concentrated urine

–  Marine birds:

 

•         Nervous/Sensory

▫         Relatively large brains, structurally similar to reptiles

–  In mammals, a complex cerebral cortex is used for learning and memory.

–  In birds, the corpus striatum is instead enlarged, which is why most behavior is stereotypic and instinctive rather than learned behavior.

–  However, many birds do show remarkable abilities to learn, on par with some mammals.

▫         Senses of smell and taste are typically poorly developed.

▫         However, senses of sight and sound are keen.

–  Bird sight

•         Ex: Hawk

 

•         Eye: relatively large and immobile

•          Retina:

•         Rods:

•         Cones:

•         Pecten: vascularized organ attached to retina

•         Reproduction

▫         Male: testes enlarge during breeding season; most male birds lack a penis

▫         Female: usually only left ovary and oviduct develop while those on right side dwindle to vestigial structures

–  Egg Production

•         Eggs are discharged from ovary and sent to oviduct.

•         As egg travels down oviduct, albumin is added.

•         Further down the oviduct, the shell membrane, shell, and shell pigments are secreted around the egg.

•         Fertilization must occur before albumin, shell membranes, and shell are added.

•         Mating Behavior

▫         Most birds are monogamous:

–  Some partner for life and remain together through years.

–  Most are seasonally monogamous and live alone for the majority of their lives.

▫         Female birds choose males that can offer high parental investment to young.

–  A male must often acquire a territory before he can mate.

–  Male sings to attract females and to discourage rival males from entering space.

–  Females visit territories, seeking the best male/territory.

▫         A few bird species are polygamous, mating with more than one individual during the breeding season.

–  Males will gather on a display ground, called a lek. Each male defends a small portion of the lek, while trying to attract females.

–  Females choose the dominant males on the lek, likely because social rank correlates with genetic quality.

•         Nesting/Parental Care

▫         All birds lay eggs that must be incubated by one or both parents.

▫         Most birds build nests to rear young, the size and shape of which is highly variable.

▫         Newly hatched birds can be altricial or precocial.

–  Altricial:

 

–  Precocial:

 

 

–  Young of both types require care from parents after hatching.

•         Migration

▫         Many birds migrate (about 4,000 species), particularly those in temperate climates. Most are south-in-winter and north-in-summer migrants.

▫         Trips can be long and strenuous, requiring extensive amounts of time and energy.

•         Stimulus of Migration:

▫         Lengthening days of late winter and early spring stimulate the development of gonads and accumulation of fat.

▫         This happens as a result of increased production of pituitary gonadotropic hormone, which causes many physiological and behavioral changes.

•         Migration navigation

▫         Most birds navigate chiefly by sight, recognizing topographical landmarks and following familiar migratory routes.

▫         Some evidence exists for ability to travel based on earth’s magnetic field.

▫         Some birds can also navigate by sun-azimuth coordination.

Mammals

•      Mammals have hair.

–     Used for protection and insulation

–     Can also be used for sensory function

–     Structure of hair

•      Grows from a follicle that is sunk into the skin

•      Rapid proliferation of cells results in growth

•      Hair is pushed upward, cells are carried away from source of nourishment and die, turning into keratin

–     Different types of hair exist

•      Under hair: soft, used for insulation

•      Guard hair: usually coarse, protection against wear, used for coloration

–     Molting

•      Some hair continuously grows to long lengths.

•      Some hair remains short.

•      Hairs are lost when new growth starts, pushing old hair out. 

•      Most mammals molt at least once a year.

–     Vibrissae

•      Vibrissae (whiskers) are modified hairs that provide tactile sense to many mammals.

•      Movement generates sensory nerve impulses that travel to special sensory areas of brain

–     Hairs can be modified into spines.

•      Other mammalian traits

–     Thick skin with relatively thin epidermis.

•      Epidermis is thickened in areas of constant contact with substrates, such as feet and palms in humans.

•      Ischial callosities: in many Old World primates

–     Horns and Antlers

•      True Horns: sheaths of keratinized epidermis over a hollow core of bone arising from the skull

•      seen in ruminants such as cattle and buffalo

•      not normally shed

•      Usually found in both males and females

•      Antlers: solid bone when mature, but are shed annually

•      Found in members of the deer family

•      Usually only present in males

•      Rhinoceros horns: composed of hairlike filaments that arise from dermal papillae and cement together to form a single horn

•      Similar to hair, but not true hairs

–     Glands

•      Sweat glands: simple, tubular, highly coiled

•      Eccrine: watery sweat in hairless regions

•      Appocrine: larger than eccrine with more winding ducts; open into hair follicles and develop at sexual maturity; often restricted to certain areas of body

•      Scent glands

•      Function and locations vary

•      Used for communication

•      Sebaceous glands: associated with hair follicles

•      Expel sebum, a greasy mixture that keeps skin and hair pliable and glossy

•      Mammary glands: likely modified appocrine glands

•      All female mammals have mammary glands

•      Rudimentary in males

•      Foods and feeding

–     Teeth are indicative of diets in mammals.

•      Almost all mammals have teeth.

•      Most mammalian teeth are heterodont: teeth are differentiated to perform specific functions

•      Different from reptile teeth, which are homodont: all similar in size and shape

•      Mammal teeth

•      Incisors: simple crowns, sharp edges for snipping or biting

•      Canines: long, conical crowns for piercing

•      Premolars: compressed crowns with 1-2 cusps for shearing and slicing

•      Molars: large bodied, variable cusp arrangement for crushing and grinding

•      Mammals usually grow 2 sets of teeth.

•      Deciduous: milk or baby teeth, include incisors, canines, and premolars

•      Permanent: grow once skull is large enough to accommodate a full set. Molars are permanent teeth.

–     Dietary categories

•      Insectivores feed on insects.

•      Small, pointed teeth

•      Relatively simple digestive systems

•      Often small bodied mammals

•      Ex: moles, shrews, bats, anteaters

•      Herbivores feed on vegetation. (Problem? Difficult to break down plant cellulose)

•      Reduced canines with broad, high crowned molars

•      Ex: cattle, elephants, gorillas, rabbits

•      Enlarged digestive tracts

•      Foregut fermentation: occurs in ruminants with large, four-chambered stomachs; regurgitation of cud

•      Hindgut fermentation: occurs in other herbivores; enlarged cecum at distal end of digestive tract; coprophagy 

•      Carnivores: feed on herbivores

•      Biting, piercing teeth

•      Powerful clawed limbs

•      Short, simple digestive tracts

•      Ex: lions, tigers, dogs, foxes

•      Omnivores: eat both plants and animals

•      Variable dentition and digestive systems

•      Pigs, raccoons, bears, many primates

•      Reproduction

–     Most mammals have definite mating systems. Mammals are typically polygamous, whereby males attempt to mate with as many females as possible, but many other mating strategies exist.

–     Females have estrus cycles and receive males for mating during breeding cycles.

–     3 patterns of reproduction in mammals:

•      Oviparous mammals: monotremes=no gestation, lay eggs, young nurse

•      Marsupials: pouched, viviparous=short gestation, long periods of lactation and maternal care

•      Eutherians: true placentals (embryo nourished by placenta), viviparous=typically long gestation, shorter periods of lactation

•      Mammals are diverse in size and form.

–     Smallest mammals are shrews and bats (3 grams)

–     Largest mammal is the blue whale (160,000 kilograms)