Signals of Spring

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Will Tweety, the Red-tailed Hawk, forge northward?

May 08, 2001 -

"It is likely that Tweety will migrate, but his/her position further north in Mexico may be one reason he's starting late- he/she may not have as far to travel to a breeding territory. Also, since we don't know his/her breeding area yet, that's another possibility for delayed migration- not having to travel very far so waiting it out on the wintering grounds. The opposite could also be true- that he/she is breeding far to the north where the weather is likely still to cold to initiate a nest. The sex of the redtail could also influence migration timing.

"In general, males arrive at breeding territories earlier than females, so Tweety might be a female, waiting for her mate to establish or re-establish last years' territory. Age and breeding status could be another reason for differential migration timing. Young birds without breeding territories may delay migration, or adults without a mate or territory may migrate early to try and establish a territory. In short,there is a lot we don't know about the timing of migration, particulary when we don't know the sex or breeding status of our birds. Our 2 redtails from last year, transmitter #s 26310 and 26313 have already shown different timing than last spring. We suspected 26310 was not breeding or bred unsuccessfully last summer. Spring migration last year began on April 5, 2000, while migration this spring began on March 18, 2001. This difference was surprising, but not unexpected considering the bird probably did not have a breeding territory last year. We suspect 26313 was breeding last year, and this bird began migrating on March 16, 2000. This season, migration began on March 28, 2001. One possibility for the later migration this year compared to 2000 could be weather related.

"In general, spring 2000 was very mild, and this spring is proving to be much wetter and colder, suggesting birds may be delaying spring migration this year compared to spring 2000. We will have to wait just a bit longer!"

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