Mystified by the Marbled Murrelet

TRU Environmental Sciences Seminar

January 20, 2011 3:30-4:30

Alan E. Burger

Department of Biology, University of Victoria

"Managing a threatened species with widely differing foraging and nesting habitats: The importance of understanding population limitation in Marbled Murrelets".

Image from: www.sierraclub.bc.ca/endangered-species

Dr. Alan Burger has studied the marbled murrelet (Brachyramphus marmoratu) for 20 years but says he still has a lot to learn. The distribution of this sea bird encompasses Alaska and the North American pacific coast where they spend roughly 80% of their life foraging underwater in near shore habitat and 20% nesting on mossy platforms of old-growth conifers.

According to Burger, copious amount of research on habitat parameters has been carried out and results show forest age, tree height and distance from ocean to be of importance. Roughly 1/3 of nestlings fledge and predation is the primary cause of nest failure. Despite the parents best attempts at hiding their offspring, predators such as crows, steller’s jays, squirrels and mice effectively stumble across eggs and young chicks. Marbled murrelets are long-lived and this coupled with low reproduction rates equates a low population turnover.



So why is the marble murrelet threatened? A closer look into marine and/or terrestrial factors limiting population size may be the answer. Burger discussed that at sea the limiting factor in population growth is prey availability while on land it’s nesting habitat, predation and social factors. The evidence is contradictory. Research from South West Vancouver Island, such as at-sea-counts and juvenile recruitment, illustrates marine factors to effect populations in the short-term. Radar counts at watersheds tracked low variability across years but high spatial changes amongst sites. In California there is a sink population (reproductive output is inadequate to maintain the local population levels) and prey availability effects breeding in some years.  However, there is immigration so food cannot be limiting. Watersheds where logging occurred showed a decrease in population size while the opposite was seen with intact forests. In Alaska, where terrestrial studies are lacking, researchers blame prey scarcity.  

Population loss is linked to logging, especially in BC and USA, says Burger. However, if we look at habitat availability, present data shows about 10-200 trees with platforms ha^-1 and an average of 1 nest per 33ha. It is not the trees with platforms that are limiting. Why then is the population struggling? An explanation:  The birds want to be spaced out. Flight and vocalization could be presented as spacing behaviour to reduce chances of a predator locating the nest. Further studies on social behaviour of these birds (although an unusual parameter in forest management) may be the key to understanding population decline of the marbled murrelet.   

My opinion:  A great lecture filled with valuable information. But we have much to learn. It’s interesting to see an example where the species’ niche is not the direct limiting factor in population reduction. A situation where the assumption of habitat loss is not to blame and there is a deeper-seeded issue to decipher.

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