Reproductive Tracts

Male reproductive system:

• internal testes: sperm production
• in most species, no penis (exceptions: ratites, waterfowl, a few other species)
• cloaca may become enlarged into cloacal protuberance

Female reproductive system:

• single active ovary (generally derived from left ovary) produces ova (eggs)
• top of oviduct: infundibulum, where fertilization may occur
• lower in oviduct, albumin and then egg membranes and shell are added
• egg laid via cloaca

Nests: variation in location, materials, shape, size, construction

• scrapes (ex: Kildeer)
• on floating platforms (ex: grebes)
• cup nest on ground (ex: dark-eyed junco)
• covered cup (woven roof) on ground (ex: eastern meadowlark)
• cup nests made of various material, in various locations (ground, on branches, in forks of trees) (ex: American Robin)
• mud nests (ex: the suboscine group called ovenbirds; some swallows)
• in excavated holes (ex: Pileated Woodpecker)
• in preexisting holes (ex: Eastern Bluebird)
• no nest -- brood parasite; lay eggs in nest of other species

Egg color

• hole nesters: often white
• ground nesters and some cup nesters-- often speckled
• blue eggs -- function not clear; often occur in species with cub nests in forks of trees

• some typical numbers:
• 1 or 2 eggs -- many shorebirds, seabirds
• 4-6 (or more) eggs -- many cup-nesting passerines
• 6-10 -- many hole-nesting passerines, woodpeckers
• up to 12 -- many waterfowl
• up to 20 -- some Galliformes
• reasons for clutch size:
• Lack's optimal clutch size hypothesis: adapted to produce the maximum number of surviving offspring -- too many eggs, not enough resources to feed young; too few, could feed more
• Tradeoff hypothesis: have fewer young than would maximize surviving offspring in a year to save energy, increase chance of survival and high reproduction in next year (more lifetime offspring)
• Predation hypothesis: have fewer eggs to decrease risk of predation -- larger clutches take longer to lay so longer vulnerable nest time; larger clutches more obvious.  Likely reason for why hole-nesters (lower predation) have large clutches than related cup-nesters (higher predation)
• Seasonality hypothesis: clutch size should reflect resources available depending on season and population size (higher populations will leave fewer resources per individual); clutches vary within an individual from season to season based on these conditions.

• incubation may be by male, female, or both
• incubating individuals develop a brood patch: featherless, fluid-filled area for heat transfer
• eggs may be cooled (by shading or wetting) in hot environments
• moundbirds: incubation by decomposing material in mound

• altricial vs. precocial (and stages in between):

• may be reached in less than a year, or not for 4 or more years
• most common: sexually mature at 1 year old.