Hypotheses:
H1: The timing of leafout is primarily a function of temperature, specifically the accumulation of degree days (thermal sum).

H2: The time when birds initiate reproductive activity (indicated by singing) is primarily a function of photoperiod but is also influenced by the timing of leafout.

H3: The phenology of reproductive activity by migratory birds is mainly a function of photoperiod while the phenology of winter-resident birds is more sensitive to plant phenology.

If Hypotheses 1 and 2 are true, then the recent tendency for warmer springs will disrupt the historical match between tree phenology and bird reproduction, possibly with negative effects on the birds. If Hypothesis 3 is also true, then the negative effects will be greater for migratory birds than for winter-resident birds.

Full research plan
Instructions for using GPS.   Data sheet for GPS waypoints.
Data sheet for leaf measurement and bird observations. 

Pictures and song recordings of focal birds from Cornell Laboratory of Ornithology. 
*American Robin
*Northern Cardinal
Eastern Phoebe
Winter Wren
Hermit Thrush
Veery
Ovenbird
White-throated Sparrow
Song sparrow

Historical data on bird phenology near Hanover, NH

Plant phenology predictor from Ohio State University
Sample zip codes for seeing the predictions: Cleveland (northeastern) = 44145; Wooster (east central) = 44691; Athens (southern) = 45701

Network of phenology gardens in Europe

Slides from Earth Day Talk, April 2007: "Climate change: science, uncertainty, and society"