Hi All,

Last year I ran an experiment where I measured the behaviour of a songbird (how long it took them to approach a speaker playing their song) in urban and rural sites along a north south transect covering 20 cities in the UK. I was interested to see if the behaviour of birds differs according to how far north they are (latitude) and if they live in an urban or rural habitat.

My results show a significant effect of latitude and also a significant interaction between latitude and habitat (latitude X urban/rural).

The results were:

Latitude: P = 0.001

Habitat: P = 0.78

Latitude*Habitat: P = 0.007

Where habitat is a factor with two levels (urban/rural) and latitude is continuous.

I used the package glmmADMB in R with the ID of the birds as a random effect to control for repeated measures. As my data are overdispersed I used a negative binomial distrubtion and a log link.

My question now is how should I interpret the interaction term? Is it acceptable to run separate tests on urban an rural birds to determine if the slopes for each group are significant?

If I do this I find a significant result for urban birds and a non-significant result for rural birds. When I plot this on a graph you can see that the slope for urban birds is steeper.

Alternatively should I just state that there was an interaction which suggests that latitude affects the behaviour of urban and rural birds differently?

Last year I ran an experiment where I measured the behaviour of a songbird (how long it took them to approach a speaker playing their song) in urban and rural sites along a north south transect covering 20 cities in the UK. I was interested to see if the behaviour of birds differs according to how far north they are (latitude) and if they live in an urban or rural habitat.

My results show a significant effect of latitude and also a significant interaction between latitude and habitat (latitude X urban/rural).

The results were:

Latitude: P = 0.001

Habitat: P = 0.78

Latitude*Habitat: P = 0.007

Where habitat is a factor with two levels (urban/rural) and latitude is continuous.

I used the package glmmADMB in R with the ID of the birds as a random effect to control for repeated measures. As my data are overdispersed I used a negative binomial distrubtion and a log link.

My question now is how should I interpret the interaction term? Is it acceptable to run separate tests on urban an rural birds to determine if the slopes for each group are significant?

If I do this I find a significant result for urban birds and a non-significant result for rural birds. When I plot this on a graph you can see that the slope for urban birds is steeper.

Alternatively should I just state that there was an interaction which suggests that latitude affects the behaviour of urban and rural birds differently?

Last edited: