Effect of food predictability on the energy budget of wintering great tits (Parus major) in southern Sweden

Abstract

Birds living in temperate regions face an energetically demanding period in winter, when food availability becomes reduced and unpredictable. In order to conserve energy, wintering passerines reduce their metabolic rate (MR) and body temperature (Tb) during the night to enter into a state of hypothermia. The effect of food predictability on the energy budget of birds is not fully understood. Therefore, we studied the energy budget by concurrently measuring and comparing nocturnal MR and Tb at -10 °C on wild great tits (Parus major) in the following two treatments: 1) an area with constant food supplementation thereby providing the birds with predictable food availability and 2) no food supplementation where the birds would experience natural and unpredictable food. The main result of our study was that young birds from the predictable area maintained a higher Tb than young birds from the unpredictable area whereas the old birds maintained a similar Tb across the treatments. We also showed that despite the differences in predictability of food availability, birds from both treatments maintained similar body mass, fat reserves and MR. We suggest that the social status of individuals and trade-offs with other maintenance processes might have enabled birds with an unpredictable food supply to maintain MR similar to the birds with a predictable food supply.

Popular Abstract

Effect of food availability on small birds in winter

Winter is a difficult period for small birds like great tits. The survival of these small birds depends on finding enough food for daily consumption. In this study, we were interested to see the effects of food availability on their wintering strategies by measuring the metabolic rate and body temperature at
-10 °C of wild great tits.

Birds living in temperate regions are faced with a challenging period in winter when ambient temperature and food availability reduces. Managing this reduction in food availability is particularly challenging for small sized birds as they cannot carry large fat reserves and must rely on finding food everyday to survive. These small birds also require lots of energy to maintain their high metabolic rate and body temperature.

What we did?
To understand how food availability affects small birds in winter, we compared the metabolic rate and body temperature of wild great tits in the following two categories: 1) an area where we constantly fed birds using feeders (see picture) during winter (fed area) and 2) an area where we did not fed the birds where they experienced a natural food availability (unfed area). Next, we caught birds from these two categories and implanted a small tag in their neck. This tag enabled us to measure their body temperature using a reader and to help individual identification.

Measuring metabolic rate and body temperature at -10 °C:
Metabolic rate is the rate of energy use by an organism and includes the activity of every tissue and process required for normal body function. We measured the metabolic rate as the overnight oxygen consumption of the birds. A few weeks after tagging birds, two birds per night from the two categories were transported to the university and placed in a respirometer which measures their oxygen consumption (therefore metabolic rate). Body temperature was also concurrently measured. These measurements were carried at -10 °C to mimic a severe winter night.


What we found?
First, we found that young birds from the fed area maintained a higher body temperature than young birds from the unfed area. Small birds maintain a high body temperature only if they have enough energy levels. The young birds in the fed area with a constant food supply could therefore maintain a high body temperature. Second, we found no difference in body temperature between old birds across the two categories. Old birds generally have prior access to food resources over young birds. Therefore, the old birds in the unfed area with their priority access to food might have been able to maintain a body temperature similar to the fed old birds.
Coming to the metabolic rate, we found that the birds from both categories maintained a similar metabolic rate. We suggest that the unfed birds might be diverting energy from other body processes such as immune system or DNA repair systems to maintain metabolic rates similar to the fed birds. To our knowledge, this study is to demonstrate the effects of food availability on the metabolic rate and body temperature of a small wintering bird.

Applications of this study:
Feeding birds during winter is a popular form of human-animal interactions. Research studies have shown both positive and negative effects of feeding birds during winter. This study helps further understand the effects of feeding birds during winter.

Master’s Degree Project in Biology 60 credits 2019
Department of Biology, Lund University

Advisors: Dr. Hannah Watson, Dr. Johan Nilsson and Prof. Jan-Åke Nilsson
Life History and Functional Ecology group, Department of Biology, Lund University

• author: Anand, Sachin
• supervisor: Hannah Watson; Johan Nilsson; Jan-Åke Nilsson
• organization: Degree Projects in Biology
• course: BION03 20182
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8989801
• date added to LUP: 2019-07-03 15:14:26
• date last changed: 2019-07-03 15:14:26