fat worries of common redshanks

james gan senior conservation officer highlights the fat issue of the common redshanks

Ever felt fat and heavy? For many people, becoming fat is not welcomed but for many birds it is crucial for their survival and vital if they are to eventually reach their breeding grounds to successfully reproduce and raise young.

The absolute weight and proportion of fat in migrant birds, such as the Common Redshank (Tringa totanus) fluctuate like a yo-yo throughout the course of a year. Lean in one month, a few weeks later, considerable weight can be put on mostly in the form of fat. Why would this happen? In a word—migration.

The fuel used by migratory birds for their journey is fat. Fat has many advantages as a fuel. It can provide twice as much energy per unit mass than any other biochemical fuel available (provides about 8 times the energy as provided by protein).

The field data, all of which were collected during bird ringing sessions, have contributed a bit more towards the understanding of the migration patterns and weight gain strategies of not only the Common Redahank but other migratory shorebirds. In particular, we have learnt more about the varying weights of the Red Shanks throughout the year and have obtainedsome indication of their weights close to their departure from the park during the northward migration. The information collected is invaluable tor the conservation management of Sungei Buloh Nature Park.

Laid down predominantly under the skin, fat is also deposited around the liver, the gut and between the wishbone. Additional weight gain also comes from the increase in the mass of the flight muscles. It is thought that the increase in muscle mass is in preparation for the strenuous long haul flight and also serves as a protein reserve for the energy demands of courtship and egg-laying at the breeding site.

The weights of Redshanks from a sample of about 370 (of birds measured between Jan 2000 & April 2001 in Sungei Buloh Nature Park) ranged from 79g to 178g with the 3 lightest birds at 79g, 85g and 87g. These lean birds, interestingly all juveniles, were taken in late Sep and early Oct during the autumn migration and may have just arrived at the park to take a break from their journey to a destination further south.

The 3 heaviest birds of 172g, 176g and 178g on the other hand were all recorded in early April and would likely be ready to depart for their breeding grounds that are thought to be in Tibet, Mongolia and Eastern Russia. Slightly more than 90% of the sampled birds had weights ranging between 100g and 140g for all periods of the year.

Maximum weights in general did not exceed 140g between Aug and Feb and there is evidence to suggest that the weights of Redshanks over-wintering in the Park remained stable during that period.

Records of Redshank weight suggest that fattening begins in March and continues in April. With one exception, Redshanks with weights above 140g were recorded only in March and April. The birds, apart from appearing fat, had also acquired varying degrees of summer plumage, another indication of reproductive preparation.

Some Redshanks from the sample have shown that they are capable of rapid weight increase. For example, weight gains of 18g and 23g have been recorded in autumn (Sept) for some juveniles over a period of 15 days in both cases.

More spectacularly, another juvenile weighing 98g that was ringed on 26 Sept and subsequently retrapped on 12 Oct had gained 28g over 16 days, an increase of 29% over its first recorded weight. If you are 60kg, imagine gaining weight at the rate of 1kg a day and ending up about 77kg in just over 2 weeks!

The retrapped Redshanks and their weight records support suggestions that some birds remain for a certain period in the Sungei Buloh area to replenish fat reserves used during the migratory journey south to escape the harsh winter conditions in the north. The evidence of a high mass gain rate also suggests a ready availability of food and could reflect the relatively favourable ecological conditions in the area although more study is required. Comments or feedback? E-mail us.

references Barter M. 1996. Ready! Steady! Go? A Crucial Decision for the Long-Distance Migrant; An Interesting Challenge for the Investigator. The Stilt 28, pp 32-42 Brooke M. & Birkhead T. 1991. The Cambridge Encyclopaedia of Ornithotogy pp 183-184. Cambridge University Press, Cambridge Wells, D. R. 1999. The Birds of the Thai-Malay Peninsula, Vol 1. Academic Press, San Diego Science frontier/Pg 10 & 11.