HabitatClimate Change & Birds

Climate Change & Birds


The earth’s climate has been changing for millions of years. The problem now is that it’s changing 20 times faster than it used to. Unfortunately, many wildlife species can take a long time to adapt to change. Over the centuries, each species of bird has adapted to certain habitats, ranges, and sources of food sources. Severe weather events, changes in temperatures and the amount and timing of snowfall or rain can have significant effects on avian migration and reproduction, predator/prey relationships, and survival.

For example, bluebirds caught in severe weather without protected roosting locations and sources of water may die. They can also starve to death if typical winter food sources like berries are covered by freezing rain/snow.

The winter weather in southern Texas is normally mild. During the deep freeze in Texas in February of 2021, people reported finding large numbers of dead bluebirds. These kinds of losses (and natural selection) can affect a population for years. Time to recovery is increased if preexisting populations were already low. Breeding bird populations in the U.S. and Canada have already decreased nearly 30 percent over the past 50 years, mainly as a result of habitat loss.

And it’s not just a winter weather issue. Bluebirds that have migrated North to breed may get caught in late spring storms. Hurricanes or severe cold weather in the peak or late part of the breeding season can wreak havoc on nesting. Droughts can influence food supply and hatch and survival rates.

Of course, weather was impacting bird populations before we starting worrying about climate change. Climate has played an important role in short term bluebird population declines and natural selection – e.g., winter freezes in the South in 1895-96, 1939-40, 1950-51, 1957-58, and severe winters in 1976-77 and again in 1977-78.

Alas, most climate models predict it’s going to get worse. Scientists expect an increase in the frequency and intensity of extreme weather events, heavy rainfall and floods, droughts, heat waves or cold snaps, and severe wildfire seasons. If the earth continues to warm at the current rate, a recent report by the National Audubon Society warns that more than two-thirds of North American species could become extinct. (National Audubon Society, 2019.

There is also an issue of timing mismatches. If environmental conditions change, a species’ distribution and the timing of life-cycle events may end up out of whack, which is called phenological miscuing. For example, if insect eating birds migrate AFTER a big hatch of insects they depend on, they may not have enough food to thrive or successfully raise a family.

Depending on the species, the timing of migration and nesting may be driven by photoperiod, but also by temperatures and food supplies.

Being able to migrate helps birds be more resilient than resident wildlife. In northern areas, and high-elevations, most bluebirds migrate, probably as a result of natural selection. The birds that did not migrate may have died due to long periods of cold temperatures and lack of food. In southern areas, populations are mostly residents, with some “partial migrants”. When these populations are hit by an extended period with subfreezing temperatures, with lots of snow and ice, they may not survive.


While it can be difficult to establish cause and effect, we know that weather can affect:

  • The cycles birds go through, from molting, migration, pairing, nest building, egg synthesis, feeding nestlings, winter fattening, to metabolic rates – e.g., colder weather will result in more energy use to maintain body temperatures)
  • Foraging conditions
  • Breeding success (chilling or starvation of young)
  • Timing and abundance of food supplies – mismatch for example, birds raised during peak prey abuandance are usually heavier and have increased survival rates. If they eat caterpillars that eat tree buds before pupating in the soil, there is a narrow window. Snow cover or freezing rain on berries
  • Timing of migration.  For long distance migrants, timing arrival is based principally on day length or endogenous clocks, and they have no way to judge weather conditions of stops
  • Breeding performance (egg size, nesting success)
  • Population sizes (which can affect the availability of mates)
  • Population distribution/geographical range
  • Habitat quality – e.g., if wildfires incinerate vegetation. If areas burn over and over, they may not recover.
  • Changes in the range of other species, including competitors, predators and parasites
  • Species interactions
  • Availability of nesting sites
  • Changes in the length of the breeding cycle (e.g., if snow cover at higher altitudes is an issue or whether they breed based on photoperiod) – what cues do they rely on? e.g., temperature, rainfall, food supplies.

Survival can be affected by factors such as:

  • How long the severe weather lasts (e.g., Cliff Swallows will die if severe weather (very cold temperatures and rain) lasts four days or more [Brown, 1999.])
  • Thermal tolerance. Bluebirds can conserve energy through “nocturnal hypothermia,” lowering of body temperature by about 10-15 degrees to conserve energy on cold nights. Metabolism (e.g., heart rate and respiration) also slow down. Roosting also helps them use 13-28% less energy (Cooper, 1999, Kendleigh 1961)
  • Fasting endurance
  • Availability of shelter (i.e., roosting in nestboxes or cavities)communal roosting. (The most bluebirds recorded roosting in one cavity was 16 birds [Pitts, 1976.])There is concern that suffocation can happen when large groups roost together in tight spaces, but I suspect hypothermia or starvation are more likely causes of death in these situations.
  • Whether the extreme event occurs during nesting season (resulting in the loss of eggs or nestlings)
  • Dispersal ability (to move with changes in climate) and wehther or not they migrate
  • How big their ecological niche is (are they geenralists/adaptable or very specialized – some species will be more vulnerable than others)
  • Disease outbreaks (e.g. avian malaria associated with movement of mosquitoes to warmer areas)
  • Genetics (e.g., have they evolved to breed only when food is abundant, or do they breed based on the season)
  • Pre-migratory fattening
  • How far they have to migrate (variability of migration)

While it can be difficult to determine the relationship between cause and effect:

  • Many bird species are moving north – by more than 40 miles between 1966 and 2013.
  • The timing of migration is changing
  • Breeding performance (egg size, nesting success) is affected, with fewer eggs, decrease in nestling weight, decreased survival rate
  • Breeding timing – some birds are breeding earlier.  This is particularly important because bluebirds don’t breed continuously.
  • Population sizes (which can affect the availability of mates)
  • Population distribution/geographical range
  • Changes in habitat qu
  • Invasive species increasing their range
  • Average arrival dates of migrant species advancing by an average of 8 days over the past 30 years (Cotton 2003) (The impacts of climate change on the annual cycles of birds (nih.gov)
  • Delayed departure from fall breeding grounds and early returns from wintering grounds
  • Sedentary populations becoming partially migratory, fully migratory species becoming partially migratory or sedentary (Berthold 2001)
  • Mountain bluebirds may be the bluebird species most at risk, as they are already losing range.

More Information and References:

  • BROWN CR AND CR BROWN. 1999. Natural selection on tail and bill morphology in Barn Swallows Hirundo rustica during severe weather. Ibis 151:652–659.
  • COOPER SJ. 1999. The thermal and energetic significance of cavity roosting in Mountain Chickadees and Juniper Titmice. Condor 101:863–866.
  • HUMPHREY, Q.P, The Impact of climate change on birds, IBIS 2004, 146, 48-56.
  • KENDEIGH SC. 1961. Energy of birds conserved by roosting in cavities. Wilson Bulletin 73:140–147.
  • PITTS TD. 1976. Fall and winter roosting habits of Carolina Chickadees. Wilson Bulletin 88:603–610.

If birds disappear, even just the soundscape around you is going to change dramatically.
– Brooke Bateman


Latest Articles