Bluebirds are a beloved backyard bird, recognized by their bright blue feathers and cheerful songs. As summer turns to fall, many people wonder if bluebirds continue nesting into the autumn months or if they have finished breeding for the year. The answer depends on a few key factors.
In general, most bluebirds do complete nesting by late summer or early fall. However, some individuals and species may produce late summer and fall broods depending on food availability, weather patterns, and their stage in the breeding cycle. Geographic location plays a role as well, with more northern bluebirds finishing earlier while southern populations may nest later.
Typical Bluebird Nesting Season
The typical nesting season for most bluebird species spans spring and summer. This includes the three common bluebird species found in North America:
Eastern Bluebird
Eastern bluebirds normally begin breeding in March and April across most of their range. First broods are usually finished by late May or early June. They often raise a second brood in June and July before wrapping up nesting by late July or August.
Western Bluebird
Western bluebirds follow a similar schedule. In most areas, these birds start nesting in April and finish up by late July or early August. Second broods may extend their breeding season into September.
Mountain Bluebird
Being adapted to cooler climates, mountain bluebirds nest a bit later than other species. They usually begin laying eggs in April or May, with the nesting season tapering off in August and September. Higher elevations populations may finish up even later.
During these peak breeding months, bluebirds are actively building nests, laying eggs, incubating, and raising multiple broods. But by late summer and early fall, their reproductive activities drop off across most of their range.
Why Bluebirds Nest Less in Fall
Several factors contribute to the decline in bluebird nesting and breeding behavior moving into autumn:
Shorter Days
The decreasing daylight hours of fall serve as cue for many birds that the breeding season is ending. This physiological trigger causes their reproductive hormones and energies to wind down.
Food Availability
Bluebirds rely heavily on insects to feed their young. As insect numbers decline in fall, it becomes more difficult for bluebirds to find adequate food for nestlings. This limits their ability to raise late broods.
Preparation for Migration
Many bluebird populations migrate south for the winter. By fall, their focus switches to storing up energy for migration and finding good wintering sites farther south. These activities take precedence over late breeding efforts.
Harsher Weather
Cold snaps and winter storms become more frequent by autumn. Fledglings from late broods may struggle to survive such conditions. Bluebirds are less likely to risk breeding this late in the year.
Molting
Bluebirds undergo an annual molt starting in late summer to replace old, worn feathers. Molting is taxing and they have reduced energy for simultaneous nesting duties.
Why Some Bluebirds Do Nest in Fall
While less common, some bluebirds will nest later in the year under certain conditions:
Mild Temperatures
Unseasonably warm weather in fall may allow insects to remain active and abundant longer. This provides adequate food for late nests. Bluebirds in southern regions where temperatures stay warmer longer are most likely to take advantage.
Exceptional Food Supply
Bumper crops of berries or other fruits may also support continued bluebird breeding. Abundant food resources offset declines in insect numbers, allowing adults to feed fall broods.
Double Brooding
In areas where bluebirds routinely raise 2 broods per season, adults may begin a late nest while fledglings from their second brood are still present. This pushes the nesting season later, sometimes into September or October.
Young Birds
Younger adult bluebirds in their first or second year may be more inclined to double brood or nest again in fall. Older, more experienced birds are less likely to expend resources on late nests.
Mild Winter Areas
In southern states where winter weather is consistently warm, bluebirds may nest several months longer. Suitable conditions allow them to breed successfully deep into fall and winter.
Late Nesting Attempts Are Risky
While some bluebirds do nest in autumn, these late breeding efforts are inherently riskier:
Increased Egg and Hatchling Mortality
Colder fall temperatures may chill eggs in the nest, reducing hatching success. Nestlings that do hatch are vulnerable to hypothermia before they can fully thermoregulate.
Less Time to Fledge Young
Late broods have a narrower window to complete development and leave the nest before winter arrives. Slow feather growth may prevent nestlings from fledging on time.
Greater Energy Demands
Feeding nestlings requires abundant food. If insect numbers crash early, parents may struggle to find enough calories for late broods, resulting in starvation.
Overlaps with Molting
Breeding while simultaneously molting may put added strain on adults. Their energy is divided between nest duties and growing new feathers.
Increased Predation Risks
Fall and winter bring increased danger from predators like snakes, mice, and raptors. Late nests are more exposed to harm as vegetation dies back. Newly fledged young are particularly vulnerable.
How to Support Late Season Bluebirds
While their chances of success are lower, some bluebird pairs do attempt nesting into fall. Here are tips to support their efforts:
Leave Nest Boxes Available
Don’t take down nest boxes in summer or fall. Empty boxes give late-breeding bluebirds the option to nest if they choose.
Continue Monitoring
Check boxes regularly through fall. This ensures other species don’t claim vacant boxes first. Timely monitoring also identifies active fall nests.
Provide Nesting Materials
Keep offering natural nesting materials like pine straw or pet fur. This saves energy for adults and helps insulate eggs and chicks.
Offer Supplementary Food
Put up feeders with mealworms, peanut bits, sunflower chips and fruits. This provides essential extra calories for breeding birds and developing young.
Insulate the Box
Add insulation inside the box to help retain warmth. Avoid materials that retain moisture. Single layers of insulation work better than thick wadding.
Use a Guard
Attach a predator guard below nest boxes to deter climbing threats. Ensure any guard does not obstruct the entrance.
Shelter From Weather
Position boxes out of direct wind and rain if possible. Provide roofs to limit exposure to the elements.
Bluebird Nesting By Month
Here is an overview of bluebird breeding activity through the seasons:
Month | Nesting Activity |
---|---|
January | No nesting |
February | No nesting |
March | Nest building begins; egg laying may start |
April | Peak egg laying period |
May | Incubating and feeding first broods |
June | Fledging first broods; second nests started |
July | Raising and fledging second broods |
August | Most nesting done; some double brooded adults may still breed |
September | Minimal nesting activity; a few late nests possible |
October | Rare late nesting attempts in warm areas |
November | No breeding activity |
December | No breeding activity |
The Odds of Fall Nesting Success
While nesting late in the year is always risky, here are some estimates for bluebird fall breeding success:
September
10-30% chance of successfully fledging young from September nests in most regions. Warmer southern areas see up to 50% success rates.
October
Only a 5-10% success rate for most October nests. Few nestlings survive to leave the nest this late in subtropical and tropical areas.
November and December
Extremely rare. Less than 5% of November or December nests successfully fledge young. Only possible in mildest climates.
So while it is not unheard of for bluebirds to breed in September or even October, the odds of fledging offspring are low and decrease rapidly as winter nears. The observations of late season nests are more notable for their rarity than their success rate.
Conclusion
While bluebirds predominantly nest in spring and summer, some individuals will attempt breeding into fall. However, factors like declining food, harsh weather, and molting make late nesting difficult. A few bluebirds manage to pull off successful autumn broods, but most have finished reproducing for the year by September. Supportive conditions like warmer weather and abundant food may extend the breeding season minimally. But bluebirds are genetically hardwired to curtail reproduction in fall to ensure their survival over winter. Though sightings exist, even in the warmest climates successful bluebird nesting remains very rare past October.