Nest failures occur for a variety of reasons
In last week's article I mentioned how frequently I was seeing the male and female blue grosbeaks at Jay's Bird Barn. Interestingly, the day the article ran, I did not see the blue grosbeaks at all. However, I did see a female brown-headed cowbird near the area of the blue grosbeak nest. By Monday of this week, the suspense of not knowing what was going on with the nest was too much, so I checked on the nest for the first time since it was originally discovered.
What I found was discouraging.
The nest had been disturbed, and was tipped up on its side, a full 90 degrees from its original position. There were no eggs, nor young. What happened to the eggs and to the nest remains a mystery, but there are several possibilities. Perhaps a cowbird was trying to lay her eggs in their nest, and the ensuing struggle with the blue grosbeak parents uprooted the nest. Maybe a cat got at the nest. Or perhaps there was some unintentional disturbance from humans, since the nest site was situated on the nursery grounds. Regardless of what happened, their nesting attempt was a failure. However, blue grosbeaks consistently raise two broods per nesting season, so we can hope that their second attempt will be successful.
Unsuccessful nesting attempts are common and occur for many reasons. As mentioned earlier, one of the reasons for failure is disturbance from predators. In the Prescott area there are many birds that will eat both bird eggs or baby birds. Species such as ravens, Western scrub jays and roadrunners are examples of birds that frequently attack nests. Bird nests are also subject to predation by raccoons, skunks, cats, snakes, and disturbance by humans! There are a lot of hazards to being a bird egg or a baby bird!
The issue of parasitism is also a challenge for songbirds. Both brown-headed and bronzed cowbirds do not build their own nest, but lay their eggs in the nest of a host species. It has been documented that brown-headed cowbirds have laid their eggs in the nests of over 200 species of birds! Some bird species are particularly vulnerable to playing host, such as warblers and vireos. When parasitism by cowbirds occurs in a nest, the effect on the host young can be very harmful. Cowbird eggs tend to hatch earlier than the host species eggs, and they also tend to grow faster. Young cowbirds will often crowd out the host species young or at least reduce their food intake.
One of the more amusing sights in the bird world is to watch the host parents caring for the much larger cowbird upon leaving the nest. The baby cowbird will flutter its wings and beg for food while the host parents are working overtime to find enough food to support the voracious appetite of "their" young, which are almost twice the size of the "parents".
On a more positive note the young hummingbirds in the nest on the nursery grounds are progressing well. Earlier today I could see the bill of one baby hummingbird protruding beyond the edge of the nest. The bill is approximately 3/4 of an inch in length, which is a sign of their growth and development. I imagine it will be at least another week before the young will be ready to fledge and leave the nest. I encourage you to take a peek at the nest when you come by Jay's; mornings are still the best time for viewing.
Interestingly enough, on the same limb where the hummingbird nest is situated, a pair of American robins have just finished building a new nest this past week. Looks like we have a little bird nursery in the nursery!
If you have specific questions, or issues related to wild birds that you would like discussed in future articles, you can submit them to Jay's Bird Barn, P.O. Box 11471, Prescott, AZ 86304, or e-mail your questions to email@example.com. Until next week, happy birding!
Eric M. Moore is the owner of Jay's Bird Barn located at Watters Garden Center; he has been an avid birder for close to 40 years.