This was a nest on my forsythia wreath on my front door this spring. Three of the eggs belong to a purple finch, the spotted egg belongs to a brown-headed cowbird. This is a classic example of brood paratism, where one species only lays their eggs in the nest of another species. The host species will either recognize the strange egg and abandon the nest, or it may incubate it along with the others. The cowbird receives the adaptive advantage of no parental investment in rearing young, where it can continue to feed and reproduce with no cost but producing the egg. Additionally, the parasite deposits the eggs of one clutch in several different nests, thereby reducing the chance of its entire clutch being obliterated by a predator. The host would gain nothing from rearing the egg, therefore it is a negative symbiotic relationship for them, and there would be strong selective pressure for them to recognize parasitic eggs.
Interestingly, in the case on my front door, the pair of purple finches incubated the cowbird egg along with the others. The cowbird hatched first, three days first according to average purple finch incubation time, and got a head start in growing. It was already large upon hatching and the finches tried to support it with a diet of seeds, the food they would feed their own young. The cowbird did not thrive, and the pair of finches abandoned the nest prior to their own eggs hatching. I am not sure if the cowbird did not thrive because its metabolic requirements were so much greater than what the finches could support, or if it was because the cowbird’s diet also includes insects.
With regard to the cowbird strategy, the behaviors involved must be more complex than it initially seems. It must be that the female cowbird would only lay her egg in nests where there were already eggs laid; it seems there would be an increased chance in host recognition otherwise. Do they likewise choose nests of those species that do not have brood parasite recognition ability? It also seems it would be advantageous to hatch first. What would happen if the host species hatched out and got a head start on development, or left the nest prior to the parasite either hatching or being even close to fledge? Do cowbirds lay eggs in nests of species that have similar incubation times, or is the strategy to just have the shortest incubation time possible? Can cowbirds even recognize the species they are laying their eggs in by either the eggs or nest structure and material? The timing of it all amazes me.