Vail Daily column: Unwelcomed eggs in the nest
April 12, 2014
Spring is officially upon the valley. In less then a month the people that migrated here for winter will migrate to their summer homes. As the winter occupants leave the valley this spring, summer inhabitants are getting ready to make their move back to the valley. One of the most highly anticipated summer inhabitants, especially for birders, is the warbler.
Warblers will start moving back to the valley as early as April and will stay until September. While they are here they will need a place to stay, so their first task upon arrival is to build a nest, which is the female's responsibility. She cannot be too hasty in her nest building because this is where she will lay her eggs and where she and her mate will raise their young throughout the summer.
The warblers must keep a close eye on their nests because unwelcome guests will try to make it their home when they are away even for a few minutes. This is similar to the human inhabitants having security systems or other humans watch over their homes while they are away to ensure no squatters take up residence. The unwelcome guests in the warbler nest are the cowbird. If the warbler does not notice the cowbird egg before it hatches, it could mean the death of the warbler's young.
Cowbird eggs do not take as long to hatch as warbler eggs. Once the cowbird chick hatches, it is known to push the unhatched warbler eggs out of the nest, smashing the eggs and killing the chicks inside. If it does not push the eggs out of the nest, then the cowbird chick will bully the warbler chicks when they hatch because they are smaller than the cowbird chicks. Since the cowbird chick is larger, it is able to get more food from the momma bird simply by pushing the smaller warbler chicks out of the way. Sometimes the cowbird will be so pushy, he will push the warbler chick right out of the nest and since the warbler chick cannot fly, he will die.
However, death by cowbird is not the usual fate of the warbler chick. The parent warblers can usually pick out the cowbird egg from the line up of eggs. The cowbird egg is too big for the warbler's small beak to remove from the nest, so she just builds a new nest on top of the parasitized nest. The warbler nests can get up to six layers high in an attempt to rid their home of these pesky nest parasites.
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So this spring and summer, watch for the bright flashes of yellow and listen for the trills of songs that tell you the warblers have arrived. Look among the willows and other shrubs along the river's edge and see if you can spot any young cowbirds hanging out with the much smaller warbler parents. These tiny little warblers, along with many other species of birds, have had to work hard to overcome many obstacles to brighten our springtime days.
Michelle Robbins is a naturalist at Walking Mountains Science Center and a bartender at the Vilar Performing Arts Center. She enjoys long walks on the beach and mountain sunsets. Look for her this summer roaming the valley's mountain sides.