Monday, May 16, 2011
The Brood Reduction Hypothesis
Hypotheses are created from observations. Field ornithologists have a hypothesis concerning brown pelican nestling survival rates. It is referred to as the brood reduction hypothesis. Anyone with at least a small sense of bird behavior knows and is usually horrified that the smallest, weakest nestling does not survive and often dies from sibling bullying. Or as the experts describe it: "starvation, aggression and/or expulsion".
If I am going to go out of my way to paddle a canoe to areas where birds roost, loaf, breed, feed, and grow and develop, than I am obliged to learn as much as I can about them. With my observations of these pelicans for the past 3 years, I have many questions. These have led me to research brown pelican nesting and from that, I have learned a few things. I didn't expect to see only beauty and joy in nature when I signed on for this amateur pursuit of bird watching and photography. But dang if nature doesn't get more harsh and often times cruel as I learn and observe more of it.
Here's what I have learned so far. Brown pelicans of the gulf coast began breeding sometime around February. The span of time is about 4 1/2 months, beginning with 2 weeks of courtship and nesting followed by 1 month of incubation. It takes about 3-4 mon before the nestlings fledge and are no longer dependent on the adults. The breeding season varies and is greatly affected by temperature (last year's freeze delayed the season noticeably). It also appears that brown pelicans are smart enough to avoid hurricane season, enough so that the young pelicans are well fledged before a big blow.
Nestbuilding continues during incubation and after hatching. Here is a shot from mid-March this year showing a parent bringing in nest material while the other parent incubates. Both males and females incubate the eggs. Nest building is quite vigorous during this period. However, I've noticed less stick delivery going on after the hatchlings arrive.
During the first 3 months of breeding season, most of the eggs have been laid, but egg laying can continue for another 3 months. Adaptive brood reduction is the name of the game for the brown pelicans and this appears to be a parental strategy for dealing with unpredictable food supplies. Here's how it works. Within about a 40 hr period, the mother lays 3 eggs, the second egg comes about 24 hrs later and the third one about 40 hr following the first. When the second hatchling appears, the first one is 16% larger. When the third one appears, it is 33% larger! This initial size difference is a competitive asymmetry that gives the first chick a head start and much better odds of survival. The mortality rate is greater for the second and third hatched birds, they feed less and grow more slowly. In the photos below, notice the one that appears smaller than the other two. In the first photo, you can barely see the smaller bird shadowed by its larger siblings.
Brown pelicans are atricial birds, meaning the hatchlings are dependent on parents for long durations. During the first 10-12 weeks, both parents care for the babies that require nourishment and thermoregulation from them. It is during the first month that nestlings experience the highest mortality rate. Success rate of initial clutches is about 50%. Mortality rate in the nest depends on several things including the parents' success at foraging, tick infestion which can lead to nest abandonment, human encrouchment (primarily ground nests) and predators. Feeding the babies requires about 1 lb of fish per day for each nestling. Feeding is a fierce competition for the siblings as seen here.
The nestlings grow in size, and feathers begin to replace the downy white. As wings grow, nestlings exercise them by flapping them vigorously before the bird can even get itself out of the nest. Space becomes an issue as the larger siblings demand more of it, while the smaller birds hang on to whatever is leftover. The pelican must also keep its pouch flexible and does so by stretching it, as seen in the third and fourth photos below. This highly vascularized pouch is the pelican's life blood for many reasons; heat dissipation through fluttering, capturing fish, and as a breeding display.
Brown pelicans are colonial nesters, as few as 10 pairs to as many as 1500 have been observed within a rookery. Typically, there is about a 4-ft distance between nests; but from what I can see, egret nests fill in some of that empty space between, leaving very little room for the large gular-pouched pelicans. Group activity is quite robust as parents tend to their babies while many juveniles and adults hang around seemingly not attending nests. I see many pelicans in juvenile plumage among the adults and nestlings. Brown pelicans do not reach full adult plumage until about 3 yrs and typically do not begin breeding until age 3-5 yrs. But, juvenile pelicans have been known to breed (although not as successfully as their older counterparts) and I have seen juvenile pelicans flying with nesting sticks. What I observe at the rookery seems to be a combination of non-breeding and breeding juveniles mixed with the adult breeders and their young. As a result of the dense population of birds, there is plenty of interaction going on as can be seen here.
Once fledged, the pelican's mortality rate is about 70-75% during the first year, but then declines to about 15% after that. Last year in late May, I watched several young fledglings in the water and flying small distances around the nest islands. Here are a few photos of some of these birds as they learn to care for themselves.
During this trip to the rookery, I noticed several juvenile pelicans flying about. There were lots of flying adults too, some bringing nest sticks back to the nest. I also noticed that the adults no longer had the bright yellow tuft of feathers on their crowns, which are noticeable during the courtship period. Here are a few pelicans lovely in flight.
And finally, knowing that the young nestlings have a low chance of reaching full adulthood (adults, by the way, can live as long as 20 yrs), here are a couple photos to compare. The soon-to-fledge bird in the first photo has less than a 50% chance of getting through the next year. With good genes and luck, this bird will come back to this rookery next year in its juvenile plumage and will then likely grow up to be a parent one day, such as the adult in the second photo.
I can't put all this information into this blog without giving credit where credit it due. Sources of information are the following:
Blus, LJ, & Keahey, JA. Variation in reproductivity with age in the brown pelican. The Auk, 95, 1978.
National Audobon Society's The Sibling Guide to Bird Life & Behavior, 2001.
Saches, EB, & Jodice, PGR. Behavior of parent and nestling brown pelicans during early brood rearing. Waterbirds, 32, 2009.
Schreiber, RW. Nesting chronology of the eastern brown pelican. The Auk, 97, 1980.
Schreiber, RW. Growth and development of nestling brown pelicans. Bird Banding, 47, 1976.
Shields, MA. Establishment and persistence of mass hierarchies in broods of the brown pelican. Wilson Bull, 112, 2000.