In high latitude grassland habitats, altricial nestlings hatching in open-cup nests early in the breeding season must cope with cold temperature challenges. Thyroid hormones (triiodothyronine, T₃ and thyroxine, T₄) and corticosterone play a crucial role in avian thermoregulation response to cold. Investigating the endocrine response of altricial nestlings to temperature variation is important for understanding the adaptive mechanisms of individual variation in the timing of breeding in birds.

We compared nest temperature, ambient temperature, body temperature, plasma T₃, T₄ and corticosterone levels in Asian Short-toed Lark (Alaudala cheleensis) nestlings hatching in the early-, middle-, and late-stages of the breeding season in Hulunbuir grassland, northeast China.

Mean nest temperature in the early-, middle- and late-stage groups was−1.85, 3.81 and 10.23 ℃, respectively, for the 3-day-old nestlings, and 6.83, 10.41 and 11.81 ℃, respectively, for the 6-day-old nestlings. The nest temperature significantly correlated with body temperature, plasma T₃, T₄ and corticosterone concentrations of nestlings. Body temperature of 3-day-old nestlings in the early and middle groups was significantly lower than that of the late group, but there was no significant difference between the nestlings in the early and middle groups. The T₄ and T₃ concentrations and the ratio of T₃/T₄ of both 3- and 6-day-old nestlings in the early-stage group were significantly higher compared to the middle and late groups. The corticosterone levels of 3-day-old nestlings were significantly higher in the early-stage group compared to the middle- and late-stage groups.

Nestlings hatching early responded to cold temperature by increasing thyroid hormones and corticosterone levels even in the early days of post hatching development when the endothermy has not been established. These hormones may play a physiological role in neonatal nestlings coping with cold temperature challenges.

Predicting the possibility of severe effects of global warming on animals is important for understanding the ecological consequences of climate change on ecosystem. Spring is the season during which birds have to physiologically prepare for the subsequent breeding period, and unusual spring temperature rising probably becomes a heat stress to the birds which have adapted to the low spring temperature. Therefore, it is necessary to understand the physiological effect of spring warming on the temperate birds.

Using the activities of blood anti-oxidative enzymes (SOD, CAT, GPx) and the concentrations of serum immunogloblins (IgA, IgY, IgM) as indicators, we compared the anti-oxidative and immune functions of Asian Short-toed Larks (Calandrella cheleensis) captured between 10 and 15 March, 2015 and housed under conditions of 21 ℃ and 16 ℃.

The SOD activities of birds in 21 ℃ group were significantly lower than those in 16 ℃ group on all the treatment days. The CAT activities of the birds in 21 ℃ group were significantly lower than those in 16 ℃ group on the 1st, 5th, 13th, 17 treatment days. The GPx activities of the birds in 21 ℃ group were signifthicantly lower than those in 16 ℃ group on the 1st, 13th and 17th, but significantly higher on the 21st treatment day. The IgA, IgY and IgM concentrations of birds in 21 ℃ group were significantly lower than those in 16 ℃ group on all the treatment days.

This study shows that spring temperature rising negatively influences antioxibative and humoral immune functions, which indicates that spring climate warming might reduce the fitness of the temperate passerine birds which have adapted to the low spring temperature.

Timing of breeding season of temperate passerines has been considered to be adjusted to their food availability. There is little work to reveal the cell stress responses of the nestlings hatched asynchronized with the food abundance peak, which is important for understanding the physiological link between the timing of breeding and the fitness of offspring.

Using gene expression level of blood HSP70 and HSP90 as indicators, we compared the cell stress response of Asian Short-toed Lark (Calandrella cheleensis) nestlings hatched under conditions of low, mid or high food (grasshopper nymph) availability in 2017.

Nymph biomass, sample time and interaction of these two factors significantly influenced the blood gene expression level of HSP70 and HSP90 of Asian Short-toed Lark nestlings. HSP70 and HSP90 gene expression levels of the nestlings at 14:00 were significantly higher than those at 5:00. At either 5:00 or 14:00, the gene expression levels of HSP70 and HSP90 increase with the decrease of nymph biomass.

These results indicate that food availability is an important environment factor inducing cellular stress of Asian Short-toed Lark nestlings. The interactive effect of the nymph abundance and sample time on the HSPs response may be related with the daily temperature variation of the grassland. Over cell stress response may be one of physiological factor mediating the effect of food availability and the nestling's fitness.

Global warming caused trophic mismatch has affected the breeding success, and even the survival, of some bird species. The ability of birds to accelerate their reproduction onset in the warmer spring could be critical to the survival of some species. The activation of the reproductive endocrine axis in birds is a key physiological process that determines the onset of reproduction. Could birds change the HPG axis endocrine rhythm under the temperature rising condition?

Using plasma LH level as an indicator of initiation of the reproductive endocrine and artificially controlling temperature, we compared variations in the timing of activation of the reproductive endocrine axis in response to temperature in two Chinese bird species, the Eurasian Skylark (Alauda arvensis) and the Great Tit (Parus major) at both the population and individual levels.

At the population level, temperature only significantly influenced LH level in the Eurasian Skylark, and had no significant effect on the LH levels in the Great Tit. Mean LH level of Eurasian Skylarks in the 20 ℃ group was higher than that of those in the 15 ℃ group throughout the experiment. Large individual variations in the timing of peak LH levels were observed in the high and the low temperature groups of both Eurasian Skylarks and Great Tits.

These results indicate that the effects of temperature differ among species. Meanwhile, there appears to be a degree of within population polymorphism in the timing of reproductive endocrine axis activation in some species. This polymorphism could provide the variation required for bird populations to cope with the possible change of their food peak under the climate warming condition.

Physiological preparation for reproduction in small passerines involves the increased secretion of reproductive hormones, elevation of the metabolic rate and energy storage, all of which are essential for reproduction. However, it is unclear whether the timing of the physiological processes involved is the same in resident and migrant species that breed in the same area. To answer this question, we compared temporal variation in the plasma concentration of luteinizing hormone (LH), testosterone (T), estradiol (E₂), triiothyronine (T₃) and body mass, between a migrant species, the Eurasian Skylark (Alauda arvensis) and a resident species, the Asian Short-toed Lark (Calandrella cheleensis), both of which breed in northeastern Inner Mongolia, China, during the 2014 and 2015 breeding seasons.

Twenty adult Eurasian Skylarks and twenty Asian Short-toed Larks were captured on March 15, 2014 and 2015 and housed in out-door aviaries. Plasma LH, T (males), E₂ (females), T₃ and the body mass of each bird were measured every six days from March 25 to May 6.

With the exception of T, which peaked earlier in the Asian Short-toed Lark in 2014, plasma concentrations of LH, T, E₂ and T₃ of both species peaked at almost the same time. However, Asian Short-toed Larks attained peak body mass earlier than Eurasian Skylarks. Plasma T₃ concentrations peaked 12 days earlier than plasma LH in both species. Generally, plasma LH, T, E₂, T₃ and body mass, peaked earlier in both species in 2014 than 2015.

The timing of pre-reproductive changes in the endocrine system and energy metabolism can be the same in migrant and resident species; however, residents may accumulate energy reserves faster than migrants. Although migration does not affect the timing of pre-breeding reproductive and metabolic changes, migrant species may need more time to increase their body mass. T levels in resident species may be accelerated by higher spring temperatures that may also advance the pre-breeding preparation of both migrants and residents.