Molting is the process by which birds shed old, worn feathers and grow new ones. It is a normal, recurring process that is essential for maintaining healthy plumage. Molting allows birds to replace damaged feathers, maintain insulation and regulate body temperature, maintain aerodynamic body shapes for efficient flight, and display markings and colors used for camouflage and attracting mates.
When Does Molting Occur?
Most songbirds have one complete molt per year after the breeding season. The timing helps reduce energy demands, as molting is metabolically expensive. Here is an overview of when different types of birds molt:
- Passerines (perching birds like finches and sparrows) generally molt in late summer/early fall after raising their young.
- Waterfowl (ducks, geese) and shorebirds molt in late summer/early fall when not nesting.
- Raptors (hawks, eagles, falcons) molt variable times depending on species.
- Colonial seabirds (albatrosses, pelicans, gulls) molt after breeding season.
The molting period lasts anywhere from a few weeks to several months depending on the species. Large birds like hawks may take up to 6 months to complete the molting process. Small songbirds may only take 4-6 weeks. During the molt, most birds are vulnerable since they cannot fly as well with missing feathers.
What Triggers the Molt to Begin?
Molting is triggered by hormonal changes within the bird’s body that are driven by seasonal cues. Here are some of the key factors that initiate the molt process:
- Changing day length – As days get shorter after the summer solstice, the change in daylight prompts hormonal shifts that trigger molting in many species.
- Decreasing testosterone levels – After breeding, testosterone levels drop off which facilitates the molt process.
- Thyroid hormones – Thyroid hormones increase leading up to and during the molt, stimulating new feather growth.
- Environmental cues – Cues such as temperatures cooling, vegetation changes, and migration preparation can help stimulate molting.
These hormonal and environmental signals cause the feather follicles to initiate the grown of new feather filaments. The old feathers are then pushed out by the emerging pin feathers.
Is Molting Different in Tropical Birds?
Birds living in the tropics where day length and temperature changes are minimal may molt on different schedules than temperate zone birds. Tropical birds often molt based on rainfall, food availability, and other environmental cues rather than changes in day length. Many tropical birds molt more than once per year, unlike temperate species.
Type of Tropical Bird | Molting Pattern |
---|---|
Hummingbirds | Occurs throughout the year with peak after wet season |
Tanagers | Twice per year, after wet and dry seasons |
Manakins | Twice per year, before and after breeding seasons |
What Happens During the Molt?
Molting occurs in a predictable progression over the body:
- Starts at the head and works down towards the tail and wings
- Smaller body feathers and semiplumes are shed first
- Wing feathers are shed symmetrically from both wings
- Inner wing feathers molt first, then middle, then flight feathers
- Tail feathers molt from innermost to outermost
This sequence allows birds to maintain their ability to fly despite missing feathers. The body feathers provide insulation so they molt first. The symmetric wing molt maintains balance and aerodynamics. And the timed tail sequence keeps the necessary lift and steering abilities.
Stages of Feather Molt
As new feathers grow in, they go through several stages:
- Old feather sheds – The old feather is pushed out by the new growing feather.
- Pin feather emerges – A pin feather starts growing wrapped in a waxy sheath.
- Blood quill stage – The feather keeps growing but with a blood supply still attached.
- Feather unfurls – The waxy sheath breaks off and the feather unfurls to reach full size.
The bird may appear scruffy during the molt as old tattered feathers fall out before new ones emerge. But the sequence ensures they maintain flight throughout the process.
How Long Does Molting Take?
The molting duration depends on the size of the bird and number of feathers that need to be replaced:
- Small songbirds take 4-6 weeks to molt all feathers.
- Medium shorebirds may take up to 3-4 months.
- Large hawks and eagles can take 4-6 months to complete the full body molt.
The number of primary flight feathers replaced also varies. Songbirds may replace 6-8 primaries per wing side. Large raptors replace only 2-3 primaries at a time so their flight ability isn’t impacted too severely.
Do Birds Molt More Than Once Per Year?
Most temperate zone birds only molt once a year. However, some species may undergo additional partial molts, including:
- Ducks – Molt body feathers twice per year, before and after breeding season.
- Tanagers – Molt body feathers twice, wing feathers once per year.
- Parrots – Molt a few feathers continuously throughout the year.
- Penguins – Molt all at once right before winter then grow a thick insulating coat.
In most cases though, the energy demands of molting mean birds try to limit it to only once per year when conditions are optimal and food is readily available.
How Do Birds Molt Their Flight Feathers Without Losing Flight?
Birds are able to molt their critical wing and tail flight feathers without becoming grounded in a few key ways:
- They molt symmetrically, shedding the matching feather on both wings at the same time.
- They molt from the inside wing out, leaving the most critical primary feathers until last.
- Only some primaries are replaced at a time, maintaining some flight ability.
- They may drop some body weight and eat extra food to compensate for reduced flight.
Young birds replacing their juvenile feathers for adult plumage will often replace all flight feathers at once when they are still being fed by parents. Adult birds don’t have this luxury and must maintain flight to forage, so their molt must be more gradual.
What Happens if a Flight Feather Breaks or Falls Out Early?
Birds can compensate well if a small number of flight feathers are lost prematurely:
- Remaining feathers spread out to close the gap.
- They may drop some weight to reduce wing loading.
- They make minor adjustments to flapping to account for missing feathers.
However, if too many critical feathers are lost, they may become grounded for safety. Some will sneak to cover on foot and molt all flight feathers simultaneously to replace them quickly before becoming vulnerable again.
How Does Molting Affect Bird Behavior and Health?
Molting is energetically expensive for birds and causes a range of behavioral and health impacts:
- Increased hunger to fuel feather growth leads to extra foraging.
- More time spent resting to conserve energy.
- Increased risk of predation with reduced flight capabilities.
- Greater stress until the process is complete.
- Possible fluctuations in body temperature until insulation is restored.
Birds appear scruffy with odd patches of missing feathers. New growing feathers have a blood supply making the bird more vulnerable. But once the molt is complete, their plumage, flight and health are restored.
Do Some Birds Not Molt?
A few unique bird species have lost the ability to molt entirely:
- Ostriches – The largest living birds. Their plumes fray but are not shed.
- Cassowaries – Large flightless birds that also don’t molt feathers.
- Kiwis – Flightless New Zealand birds with hair-like feathers.
Without the need for aerodynamic flight feathers or seasonal temperature regulation, these birds have evolved to lose molting. Their feathers are still gradually replaced, but not in an organized molt.
Conclusion
Molting is a complex, energy-intensive process evolved to keep birds healthy and their feathers functioning optimally. It is triggered by hormonal shifts brought on by seasonal daylight, temperature, and environmental cues. Different species molt at different times based on factors like breeding, migration, and food availability. The molt follows a sequential pattern over the body to maintain flying ability. While molting can be taxing, it is a necessary part of the avian life cycle that maintains plumage condition and thermoregulation.