Date: 2/14/18 8:43 am From: AB Clark <anneb.clark...> Subject: Re: [cayugabirds-l] Bird Signs of Earlier Spring?
There are certainly more knowledgeable ornithologists to answer but this touches on some questions we are trying to answer with crows over 30 years. (Over that time, no directional change in average winter-spring temps, in part because 1990 is a VERY warm year.)
Gonadal development is typically related to day length and the direction of day length change in birds, and goes on regardless of temperatures.
There are some semi-known, semi-hypothesized mechanisms by which birds detect longer days. Essentially it goes something like this: sunrise re- sets the bird’s “endogenous” or innate rhythm of behavior and neural activities. (That is another story..) After that there is a period in which the bird is happily unresponsive to dark vs light. But that period ends about 4 in the afternoon, and after that the bird is increasily senstitive to light being present. If it isn’t, as for short days around the solstice at this latitude, the bird just goes to sleep without worrying about hormones (so to speak). BUT if the light is still there when it is sensitive later in the day, that information stimulates or begins to stimulate gonadal development. As days go by, the other part of the cue is the lengthening or later availability of light: the day is not only 11 hours long but it is 2 min longer than yesterday.
Note that birds that are spending the winter near the equator cannot be using this mechanism as a decision as to when to migrate. The circannual clock is probably involved here, although birds could then come part way and finish migration using day length.( I forget the recent literature here.) But birds that are migrating definitely don’t benefit from making big gonads to carry along on migration.
Actual decisions to move to nesting habitat, develop testes and sing or begin developing ova preparatory to laying eggs have to be more fine tuned…to weather (not climate), to personal condition and food resources, etc. So the whole thing is a layered process of information gathering, some quite codified, some quite flexible.
OK—I am no specialist in this, so I will be happy to bow to more educated answers, or to try to find answers to specific questions. For those of you who do “skulling” to age birds, that thin skull permits light to penetrate directly to the pineal gland in birds…something mammals cannot do, so they use an eye-brain connection.
PS for birds like budgerigars in Australia that breed erratically when there is rain, rain seems to cue migration to breeding grounds and greening foods (wild millet for instance) and dark nest holes spur ovarian development in females.
Anne B Clark
147 Hile School Rd
Freeville, NY 13068