In uncertain times, it makes sense to manage risk in your endeavors — whether it’s investing in money-making opportunities or deciding where to lay your eggs.
Brood parasites are birds that are known to lay their eggs in other birds’ nests. Cowbirds and cuckoos are among the most famous examples of this group.
New research from Washington University in St. Louis finds that brood parasites living in more variable and unpredictable habitats tend to parasitize — or squat and drop their eggs in — the nests of a greater variety and number of hosts. The study is published Aug. 21 in Nature Communications.
“When brood parasites face increased ecological risks — for example, greater climatic uncertainty in their environment, or greater uncertainty with regards to the availability or behavior of their hosts — they turn to bet-hedging,” said Carlos Botero, assistant professor of biology in Arts & Sciences and the study’s senior author.
“In other words, when it is difficult to predict the ideal host, parasites literally lay their few precious eggs in more than one basket,” he said. “This means increasing not just the number of different host species they use, but also expanding the diversity of taxonomic families that they choose as hosts.”
A birder himself, Botero says that he is fascinated by things animals do that fall outside the boundaries of what some think of as “typical” — like brood parasitism.
“Parasite mothers can’t really do much about the behaviors that their hosts will display as surrogate parents,” Botero said. “With bet-hedging in the choice of hosts, parasites are at least able to increase the chances that one — or a few — of the surrogate parents they choose will end up behaving in the optimal way.”
Botero and his colleagues at the University of Illinois Urbana-Champaign and Columbia University observed a pattern that they considered striking.
The researchers aggregated environmental, parasite and host species data associated with 84 species of obligate avian brood parasites from 19 genera and five different bird families. Their list covered approximately 86% of all known brood parasitic species.
For all of these birds, host behavior is critical when it comes to countering environmental threats. Even small differences in the nest architecture, habitat selection, breeding timing or incubation behavior of the chosen surrogate parents can have life or death implications for young parasitic chicks.
A brood parasite’s properly “hedged” portfolio must include a reasonable diversity of host types to ensure that at least some reproductive success is achieved — no matter what environmental conditions are experienced in any given year.
“A bet-hedging strategy involves making some or sometimes even many ‘wrong’ choices,” Botero said. “For example, for years in which the behaviors, timing and nest type of a given host clearly work better than those of other species, it would be clearly ideal to stick with that option and avoid wasting eggs on others.”
The problem is, parasites that live in variable and unpredictable environments cannot know at the onset which option will work best that year.
“Parasitic mothers that diversify their egg-laying choices may not contribute as many offspring to any given generation as they would have if they had chosen the best host type that year,” Botero said. “But, over time, they will end up contributing a much larger total number of offspring to future generations by fledging some offspring every year.
“It is this long-term vision that allows bet-hedging lineages to prevail and to steer the course of evolution so that in the end, everyone in their species bet-hedges.”
New research from Washington University in St. Louis finds that brood parasites living in more variable and unpredictable habitats tend to parasitize — or squat and drop their eggs in — the nests of a greater variety and number of hosts. The study is published Aug. 21 in Nature Communications.
A suitable diversity of host types is essential for a brood parasite’s “hedged” portfolio in order to guarantee at least some reproductive success, regardless of the environmental circumstances encountered in any given year.
Host behavior is essential for all of these birds to combat environmental threats. For young parasitic chicks, even minute variations in the design of the nest, choice of habitat, timing of breeding, or incubation style of the selected surrogate parents can mean the difference between life and death.
Botero noticed a pattern that his colleagues at Columbia University and the University of Illinois Urbana-Champaign found to be quite remarkable.
According to Botero, “parasite moms really have little control over the actions that their hosts will take in their capacity as surrogate parents.” By using bet-hedging when selecting hosts, parasites can at least raise the likelihood that one or more of their surrogate parents will ultimately behave in the best possible way. ”.
Originally from the United States, brown-headed cowbirds favor agricultural, urban, and suburban settings with easily accessible grain or cattle-disturbed soil. They used to follow herds of bison and eat the insects that the animals’ hooves kicked up. It’s unclear if they adopted their breeding strategy as a result of having to relocate regularly to stay up with the bison herds, or if their breeding strategy allowed them the flexibility to do so. This species has benefited greatly from the expansion of agricultural areas and the removal of forest cover, which have increased overall habitat and allowed cowbirds to access new host species that do not have defensive mechanisms against nest parasitism. Although it is evident that cowbirds have profited from forest fragmentation, it is less certain how much of a role they have played in the population-level declines of many forest birds.
Because cowbirds are native to the U.S., they are protected under the Migratory Bird Treaty Act and in most instances it is unlawful to use lethal control without a permit, including the removal of their eggs from a nest. However, unpermitted control of cowbirds is occasionally permissible under special circumstances outlined in the Migratory Bird Treaty Act. Additionally, in some states, such as Michigan and Texas, permits can be obtained to trap cowbirds to protect endangered species like Kirtland’s Warbler, Golden-cheeked Warbler, and Black-capped Vireo. Please check with your state’s wildlife management agency for local regulations.
Due to its ability to parasitize over 220 different species of North American birds, the cowbird is not exclusively dependent on one host species, allowing its effects to be felt by numerous populations. While cowbirds have been linked to the decline of rare species like Black-capped Vireo and Kirtland’s Warbler, habitat loss and fragmentation probably contribute significantly more to songbird declines. This is demonstrated by the fact that the endangered Kirtland’s Warbler did not see a rise in population on its own; rather, the warblers did not recover until cowbird control was paired with habitat management for nascent Jack Pine forests.
The Brown-headed Cowbird (Molothrus ater) is a brood parasite, meaning that it lays its eggs in nests of other species. A female cowbird quietly searches for female birds of other species that are actively laying eggs. Once she has found a suitable host, the cowbird will sneak onto the resident bird’s nest when it is away, usually damage or remove one (or more) egg, and replace that egg with one (or more) of her own. The foster parents then unknowingly raise the young cowbirds, usually at the expense of their own offspring (watch a cowbird fledgling being fed by an Eastern Phoebe and a cowbird removing an Eastern Kingbird nestling from a nest at 0:45). Cowbird eggs require a shorter incubation period than most other songbirds and thus usually hatch first. Cowbird nestlings also grow large very quickly. These advantages allow them to command the most food from their foster parents, usually resulting in reduced nesting success of the host species.
Some species, such as the Yellow Warbler, can recognize cowbird eggs and will reject them or build a new nest on top of them. Those species which accept cowbird eggs either do not notice the new eggs, or as new evidence suggests, accept them as a defense against total nest destruction. Cowbirds may “punish” egg-rejectors by destroying the entire nest, whereas it is possible for egg-acceptors to raise some of their own young in addition to the cowbird young (see Birdscope 2008).
FAQ
What birds leave their eggs in other nests?
Which bird lays it eggs in nests other than its own?
Why do cuckoos leave their eggs in other nests?