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Hence, maintaining body temperature (\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>b<\/jats:sub>\n                    ) in\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>a<\/jats:sub>\n                    below thermoneutrality can be energetically costly for a small bird. We still know little about how increased heat production below thermoneutrality affects the level at which\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>b<\/jats:sub>\n                    is maintained, and if these patterns are age specific. To test this, we measured subcutaneous body temperature (\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>s<\/jats:sub>\n                    ) and resting metabolic rate (RMR) simultaneously in blue tits (\n                    <jats:italic>Cyanistes caeruleus<\/jats:italic>\n                    ) during winter nights in\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>a<\/jats:sub>\n                    \u2019s ranging from 25 to \u2212\u200915\u00a0\u00b0C. RMR increased below the lower critical temperature (LCT, estimated at 14\u00a0\u00b0C) and was 6% higher in young (birds in their first winter) compared to old birds (birds in their second winter or older). The higher RMR was also mirrored in higher\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>s<\/jats:sub>\n                    and thermal conductance (\n                    <jats:italic>C<\/jats:italic>\n                    ) in young birds, which we suggest could be caused by age differences in plumage quality, likely driven by time constraints during moult. Reduction in nightly predicted\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>b<\/jats:sub>\n                    was modest and increased again at the coldest ambient temperatures, suggesting that either heat retention or heat production (or both) improved when\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>a<\/jats:sub>\n                    reached levels which are cold by the standards of birds in our population. Our results show that levels of heat production and\n                    <jats:italic>T<\/jats:italic>\n                    <jats:sub>b<\/jats:sub>\n                    can be age specific. Further studies should address age-specific differences on quality, structure, and thermal conductivity of plumage more explicitly, to investigate the role of variation in insulation in age-linked metabolic phenotypes.\n                  <\/jats:p>","DOI":"10.1007\/s00360-020-01266-5","type":"journal-article","created":{"date-parts":[[2020,2,24]],"date-time":"2020-02-24T06:07:22Z","timestamp":1582524442000},"page":"349-359","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":26,"title":["Age differences in night-time metabolic rate and body temperature in a small passerine"],"prefix":"10.1007","volume":"190","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-7631-4258","authenticated-orcid":false,"given":"Fredrik","family":"Andreasson","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6170-689X","authenticated-orcid":false,"given":"Andreas","family":"Nord","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8982-1064","authenticated-orcid":false,"given":"Jan-\u00c5ke","family":"Nilsson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2020,2,24]]},"reference":[{"key":"1266_CR1","doi-asserted-by":"crossref","first-page":"329","DOI":"10.1007\/s00442-018-04331-7","volume":"189","author":"F Andreasson","year":"2019","unstructured":"Andreasson F, Nord A, Nilsson J-\u00c5 (2019) Age-dependent effects of predation risk on night-time hypothermia in two wintering passerine birds. 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