Quick Answers
If a bird abandons its nest before the eggs hatch or the young fledge, the eggs or nestlings will likely die from exposure, starvation, or predation. However, in some cases, other parent birds may adopt abandoned eggs or nestlings. The impacts of nest abandonment depend on the species, stage of breeding, and other factors. Abandonment may occur due to disturbance, lack of resources, death of a parent, or other reasons.
Birds invest a great amount of time and energy into building nests, laying and incubating eggs, and raising nestlings. However, nest abandonment, where the adult birds desert the eggs or young in the nest, is a relatively common occurrence. This can happen for a variety of reasons and at different stages of the breeding process. When abandonment occurs, the outcomes for the eggs or nestlings are often dire. But in some special cases, other birds may step in to care for the abandoned young. The impacts of abandonment depend on the species, timing, and other contextual factors.
What Causes Birds to Abandon Their Nests?
There are a number of potential reasons why adult birds may abandon their nests:
Disturbance
Birds are sensitive to disturbance during breeding. Excessive noise, human interference, predators, or other intrusions could lead birds to abandon their nests. Some species are more prone to abandonment than others based on natural temperament and other factors.
Lack of Resources
Birds may abandon nests due to insufficient food resources or other habitat requirements. This is especially likely early in the breeding season. Birds invest less energy in eggs compared to nestlings, making them more likely to abandon at the egg stage.
Death of a Parent
If one parent dies during the breeding attempt, the other may abandon the nest. Single parents struggle to incubate eggs, keep nestlings warm, and provide enough food.
Nest Parasitism
Some birds, like cuckoos, lay eggs in the nests of other species. The host species may abandon nests with cuckoo eggs.
Infertility
Birds may abandon nests with infertile or non-viable eggs. This allows them to conserve resources and begin a new nest elsewhere.
Predation
Heavy predation pressure could lead birds to abandon nest sites or entire breeding attempts. This is especially true early in nesting before much energy has been invested.
Inclement Weather
Severe storms, flooding, or drought could decrease habitat quality or food availability and cause abandonment. Birds may attempt to re-nest elsewhere with better conditions.
Parasites and Disease
High parasite loads in nests or infectious diseases could also cause abandonment in some cases.
What Happens to Eggs and Nestlings?
The outcomes for abandoned eggs and nestlings depend heavily on the stage of breeding.
Abandoned Eggs
Bird eggs require continuous incubation and protection. If abandoned before hatching, eggs are unlikely to survive:
- Exposure kills embryos through chilling, overheating, or desiccation.
- Eggs are vulnerable to predators like snakes, squirrels, and raptors.
- Rot may quickly set in for non-viable eggs.
- Some abandoned eggs may hatch if briefly unattended, but the chicks then die.
Late stage embryos or hatchlings have very low survival chances without parental care. Overall, abandonment almost always leads to the complete loss of eggs.
Abandoned Nestlings
Nestlings may survive 1-2 days after abandonment depending on age, but require constant feeding and brooding. Dangers include:
- Starvation is the greatest threat as nestlings rely completely on parental feeding.
- Exposure kills altricial nestlings unable to thermoregulate.
- Predators like snakes, rodents, and scavenging birds consume unattended nestlings.
- Parasites may proliferate without parental nest maintenance.
Nestlings over 75% of fledging age may survive abandonment for a few days in some species. But survival rates past 1-2 days are extremely low in most species.
Will Other Birds Adopt Abandoned Eggs or Young?
While rare, there are some fascinating cases where birds adopt abandoned eggs or nestlings of their own species or even other species:
Conspecific Adoption
Some birds, like grebes, geese, and ducks, may take over parenting duties for abandoned eggs or young of the same species. This cooperative breeding improves overall reproductive success. Adoption is more likely among relatives or familiar birds.
Heterospecific Adoption
Rarer still, certain bird species have been known to adopt the young of other species. Examples include:
- House sparrows adopting tree swallow chicks.
- Barn swallows feeding Eastern phoebe nestlings.
- Eastern bluebird pairs raising abandoned house wrens.
The adoptees are often similar sized species using comparable nest sites. Stress hormones in pleading nestlings may stimulate adoption. But the fostering parents incur a high cost of feeding genetically unrelated chicks.
Brood Parasitism
The closest natural analogy to adoption is obligate interspecies brood parasitism. Cuckoos and cowbirds lay eggs in other birds’ nests, who raise the unrelated chicks – often at the expense of their own. But this relationship is parasitic rather than cooperative.
Key Factors Influencing Impacts of Abandonment
The effects of nest abandonment on eggs, nestlings, and parent birds depend on several key factors:
Stage of Breeding
Earlier abandonment of eggs reduces wasted reproductive investment compared to later abandonment of nestlings. Birds are more likely to abandon eggs than partly grown young.
Developmental Stage
NEW TABLE
Developmental Stage | Survival Time After Abandonment |
---|---|
Eggs | Hours to days |
Nestlings | 1-2 days, longer in some species |
Fledglings | 1 week or more with parental support |
Older nestlings and fledged chicks have higher survival potential if abandoned.
Availability of Other Parents
Conspecific adoption improves survival odds for abandoned young. But most birds lack this backup support.
Species Traits and Habitat
Cavity and enclosed nests offer more protection. Altricial young are far more vulnerable than precocial species. Climate extremes reduce survival.
Predation Pressure
Heavy predation makes abandoned eggs and nestlings unlikely to survive exposure. More predators may also deter adoption.
Food Availability
Scarce resources increase competition and decrease adoption likelihood. Fostering parents need adequate food for own and adopted chicks.
Survival Rates of Abandoned Eggs and Nestlings
While adoptive parents improve outcomes, overall survival rates for abandoned nests across all species remain extremely low:
Abandoned Eggs
– 90-100% mortality is typical for abandoned eggs. Embryos rarely survive to hatching without continuous incubation and care.
Abandoned Nestlings
– 60-100% mortality is typical for abandoned chicks. Altricial species suffer higher losses than precocial nestlings.
Even among fostered chicks, high mortality results from improper diet, exposure, and energy costs to adoptive parents. Most abandoned nests ultimately fail completely.
Impacts on Parents
Nest abandonment may benefit parent birds by conserving resources under dire conditions. But it also eliminates their current reproductive investment. Impacts include:
- Wasted energy expenditure on eggs, nest building, and incubation.
- Lost time that could have been used for other nesting attempts.
- Delayed future nesting and potential loss of annual reproductive success.
- Increased pressure to renest and rebuild energy reserves quickly.
- Higher predation risk if renesting in marginal or suboptimal habitat.
Despite costs, abandonment may optimize lifetime reproduction in harsh or declining environments. Still, nest failure takes a toll on breeding adults.
Why Do Birds Sometimes Abandon Healthy Eggs or Nestlings?
While abandonment of non-viable eggs reduces wasted investment, abandonment of healthy eggs or nestlings is more puzzling:
External Mortality Threats
Birds may “cut their losses” and abandon nests threatened by severe weather, lack of food, or heavy predation.
Parasites and Disease
High parasite loads or infectious diseases at nest sites could also trigger abandonment.
Perceived Risks
Birds use environmental cues to assess risks. False signals like human disturbance could inadvertently trigger abandonment.
Lack of Investment
Some birds show little attachment to eggs compared to nestlings. Greater parental investment reduces likelihood of abandonment.
Alternate Nest Sites
Birds may abandon to take advantage of other nesting options. This is rarer mid-season compared to early nesting.
Limited Resources
Scarce food may force birds to selectively abandon some eggs/nestlings to ensure survival of others.
Change in Parents
New mates or surviving parents may abandon offspring from previous pairings.
While puzzling, abandonment of viable eggs or nestlings may still improve lifetime success under environmental pressures.
Can Abandoned Nests Be Saved?
Active intervention to save abandoned bird nests is rarely successful:
- Getting substitute parents is unlikely, limiting survival odds.
- Hand-rearing requires expertise, time investment, and raising birds that lack survival skills.
- Disturbance to move or artificially incubate often exacerbates abandonment.
- Legal restrictions apply due to the Migratory Bird Treaty Act.
- Sampling for diagnostics may be needed to rule out disease risks.
Leaving nests undisturbed or minimizing impacts that caused abandonment are the best courses of action. Focus should be on protecting naturally successful nests.
Preventing Nest Abandonment
Some strategies may help conserve bird populations by reducing abandonment:
- Habitat protection limits scarcity of nest sites and resources.
- Predator removal in key breeding areas.
- Parasite control through nest sanitation, fumigation, or treatment.
- Limiting recreational impacts and human disturbance.
- Nest monitoring to identify risky situations.
- Supplemental feeding when food is scarce.
However, abandonment is a natural behavior and some level will always occur. The emphasis should be on supporting successful nests through habitat management.
Conclusion
Nest abandonment is a significant threat to bird reproductive success across many species. While abandonment is a complex behavior with many causes, the outcomes are usually dire for eggs and nestlings without continuous parental care. Still, some abandoned young may be adopted by other birds in rare cases. Understanding the pressures behind abandonment and focusing conservation efforts on properly functioning nests will be key to supporting bird populations. With informed management, the impacts of abandonment can be reduced. But it will continue as an inherent challenge during breeding under natural conditions.