Temperature change is an important departure cue in nocturnal migrants: controlled experiments with wild-caught birds in a proof-of-concept study

Abstract

The decision-making process of migrating birds at stopover sites is a complex interplay of the innate migration program and both intrinsic and extrinsic factors. While it is well studied how variation in precipitation, wind and air pressure influence this process, there is less evidence of the effects of temperature changes on the departure decision. Thus, we lack knowledge on how the predicted changes due to global climate change in temperature alone may affect the decision-making process during migration. Aiming to fill parts of this gap, we conducted a proof-of-concept study by manipulating the ambient temperature of temporarily confined wild-caught migrant songbirds under constant feeding conditions. In spring, departure probability increased with a 20°C rise in temperature for both a medium-distance migrant (European robin, Erithacus rubecula) and a long-distance migrant (northern wheatear, Oenanthe oenanthe), and in autumn, departure probabilities of the long-distance migrant both decreased with a 20°C rise and increased with a 20°C drop. Consequently, the temperature is an important departure cue influencing the decision-making process of migrating songbirds. Incorporating causal relationships between changes in temperature and departure probability in migration models could substantially improve our ability to predict the effects of climate change on the phenology of migratory birds.

Keywords: decision-making process; departure probability; migration; songbird; stopover; temperature.

Figure 1.

Experimental design. Darker colours indicate the nights during which nocturnal migratory restlessness was measured. Upon capture, birds were randomly divided into two groups: group A entered the room with an ambient temperature of 25°C (orange), group B entered the room kept at 5°C (blue). After the third night of captivity, group A was transferred to the room kept at 5°C, group B to the room kept at 25°C. Both groups received ad libitum food during the daylight period and ad libitum water throughout. Migratory restlessness was measured each night, but data of the first and second night were not considered for the main analyses because they were considered as an ‘acclimatization phase’ (for details see Methods and for data of all nights the electronic supplementary material, table S4 and figure S8). (Online version in colour.)

Figure 2.

Evening energy stores in European robins and northern wheatears were calculated using evening body mass for day 3 and 4 during spring migration. Blue boxes represent the temperature of 5°C, orange boxes the temperature of 25°C. Significant effects (table 1) are marked with an asterisk. The plus indicates the direction of the effect. The number of birds are given above the x-axis. Box plots show the 5th, 25th, 50th, 75th and 95th percentile as well as outliers (dots). For data of all nights see the electronic supplementary material. (Online version in colour.)

Figure 3.

Evening energy stores in European robins and northern wheatears were calculated using evening body mass for day 3 and 4 during autumn migration. Blue boxes represent the temperature of 5°C, orange boxes the temperature of 25°C. Significant effects (table 2) are marked with an asterisk. The plus indicates the direction of the effect. The number of birds are given above the x-axis. Box plots show the 5th, 25th, 50th, 75th and 95th percentile as well as outliers (dots). For data of all nights see the electronic supplementary material, table S3 and figure S7. (Online version in colour.)

Figure 4.

Effects of a change in temperature on migratory restlessness in European robins and northern wheatears for experimental night three and four during spring migration. Migratory restlessness is expressed as the number of active 1 min intervals per night, which was restricted to the first 10 h (see Methods). Blue dots represent the temperature of 5°C, orange dots the temperature of 25°C and are connected for each individual by a line. The number of individuals are given at the top. Significant effects (table 3) are marked with an asterisk. The plus indicates the direction of the effect. For data of all nights see the electronic supplementary material, table S4 and figure S8. (Online version in colour.)

Figure 5.

Effects of a change in temperature on migratory restlessness in European robins and northern wheatears for experimental night three and four during autumn migration. Migratory restlessness is expressed as the number of active 1 min intervals per night, which was restricted to the first 7 h (see Methods). Blue dots represent the temperature of 5°C, orange dots the temperature of 25°C and are connected for each individual by a line. The number of individuals are given at the top. Significant effects (table 4) are marked with an asterisk. The plus or minus indicates the direction of the effect. For data of all nights see the electronic supplementary material, table S4 and figure S8. (Online version in colour.)