Don’t Put All Your Eggs in One Basket: Brood Parasitism in Birds

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At the Laura Quinn Wild Bird Sanctuary, we recently came across a bird, that at first, we were unable to identify. Shortly thereafter, we discovered the juvenile bird incessantly begging an adult cardinal for food which the cardinal ended up providing. Upon further investigation of its morphology, it became clear this was a juvenile cowbird. Cowbirds are one of multiple groups of obligate brood parasites, birds that lay their eggs exclusively in the nests of other bird species. These other species raise the parasitic cowbird offspring. Intraspecific brood parasitism, another type of brood parasitism, occurs when a female lays her eggs in the nest of a bird pair from the same species. Furthermore, some birds such as the Yellow-billed Cuckoo and Black-billed Cuckoo, serve as a bridge between intraspecific and obligate brood parasitism, parasitizing each other’s nests only occasionally (i).

Though unique, brood parasitism is a highly successful reproductive strategy. At least 234 species in 16 avian orders utilize intraspecific brood parasitism, especially waterfowl (i). Obligate brood parasitism has evolved in at least 96 species around the globe, the most familiar in the United States being the Brown-headed Cowbird (i). The advantage of brood parasitism is clear; you forward the costs of parental care to another bird pair. Also, since their parasitic nestlings are spread across many nests, the risk of predation is also significantly reduced. Specifically, obligate brood parasites don’t have to waste time or energy making a nest, defending their nest from predators and competitors, and incubating and feeding nestlings. With all this free time and energy, brood parasites can spend more time mating and producing more eggs. In fact, female cowbirds lay 30-40 eggs per breeding season at 2-5 eggs per week, a number that significantly exceeds most passerines (i).

parasitized bird nest

Pictured Left: Hatched cowbird chick in a dickcissel nest with two other cowbird eggs (white and brown) and two dickcissel eggs (blue). Photo Courtesy of Kurt Gielow.


You may be thinking, how do parasitic birds get away with tricking host birds to raise their young for them? There are various strategies brood parasites have evolved to ensure their chick emerges from the host’s nest strong and healthy. Cuckoos in Europe disguise their eggs as the same color and pattern as their primary host’s eggs to prevent identification and destruction by the host parent. Cuckoo eggs also have hard thick shells which is advantageous when dropping eggs into a deep nest (i). The hard cuckoo egg will stay intact while host’s eggs are likely to crack upon impact. Hard eggs can also prevent the host parent from easily puncturing a rejected egg with its beak (ii). Eggs and nestlings of obligate brood parasites are normally the same size or larger compared to their hosts, require less incubation time, grow faster, and beg for food louder, all contributing to dominance over remaining host nestlings (i). Several hostile strategies have emerged as well. Recently hatched cuckoos will shove unhatched host eggs out of the nest as demonstrated in the Artur Homan’s video below:

The determination of the newly hatched cuckoo in this video is truly remarkable. Even more violent, nestling honeyguides have fanglike hooks at the ends of their bills for killing their rival nest mates (i).

Despite the suite of strategies employed by obligate brood parasites, some host species have evolved the ability to recognize and reject foreign eggs and nestlings based on visual and acoustic sensory cues. Female starlings will remove any eggs deposited before they themselves start to lay while some cuckoo hosts are more likely to eject a cuckoo egg from their nest if they’ve seen a cuckoo in the vicinity (i). American Yellow Warblers that have detected foreign eggs will abandon the nest or bury the entire clutch below and lay a new clutch above (i). Female coots also bury foreign eggs or move them to an inferior incubation position, delaying or preventing hatching of the intruder (i). These dueling strategies surrounding parasitic egg/nestling acceptance or rejection serve as an excellent example of an evolutionary arms race between parasite and host.

As you might have expected, the consequences of brood parasitism on host species can be severe. Most parasitized nests do not fledge host chicks due to the range of previously discussed strategies employed by brood parasites. Deforestation has further amplified the negative effects on host species. In the eastern United States, brood parasitism rates are significantly higher among Brown-headed cowbirds and their hosts in fragmented forests (iii). The endangered Kirtland’s Warbler serves as a historical example of one species whose population has been significantly affected by the synergism of deforestation and cowbird parasitism. From a conservation perspective, this highlights the importance of maintaining and creating quality habitat for birds and other wildlife.

Evolution has shaped many unique strategies for survival and reproductive success even though we may consider these strategies harsh. Brood parasitism serves as an excellent example of a peculiar yet successful reproductive strategy. By now, you can see why some birds put their eggs in the nests of others instead of making their own.

Steven Warchocki, FKWBC Intern, 2016

References
(i) Gill, F. B. (2007). Ornithology (3rd ed). New York: W.H. Freeman and Company.
(ii) Croston, R., & Hauber, M. E (2010). The Ecology of Avian Brood Parasitism.
(iii) Cavitt, J. F, & Martin T. E. (2002). Effects of forest fragmentation on brood parasitism and nest predation in eastern and western landscapes. Studies in Avian Biology 25 (73-80).

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