Convergence Problems with glmer.nb model

Good morning,

First time posting so I hope I am including all of the relevant
information.

I am attempting to analyse the foraging behaviour of a animal in an
agricultural landscape. The objective is to identify the factors (habitat
type, environmental variables and animal-specific variables) that best
predict foraging site preference. Some fields are preferred while others
are avoided.

The response variable is count data - the number of times a given animal
was in a given field in a given month. An animal's home range varies from
month to month, so the area available to it and the fields that fall within
its home range change somewhat every month. The count data shows an
overdispersed, negative binomial distribution, and is zero inflated as
fields that fell within the home range where the animal had *not *foraged
in that month are also included in the dataset. The individual animal is
specified as a random variable to account for pseudoreplication.

It should be noted that at the moment I am attempting to run a the model
on a subset of the data (n=671) as I had attempted to run the model on the
full dataset (n=62,000) but three days later the model (which included
interaction terms at this point) had still failed to run, and when stopped,
R gave me a multitude of convergence warning messages e.g.

13: In (function (fn, par, lower = rep.int(-Inf, n), upper = rep.int(Inf,
... :
  failure to converge in 10000 evaluations

Simpler iterations of the model, with fewer explanatory terms, and no
interaction terms, also gave me convergence and some scaling warnings,
which I sought to address using:

control=glmerControl(optCtrl=list(maxfun=20000)

and by scaling the numeric variables age, slope and aspect as follows:-

dframe1$agescale <- scale(dframe1$age, center = TRUE, scale = FALSE)
dframe1$slopescale <- scale(dframe1$slope, center = TRUE, scale = FALSE)
dframe1$aspectscale <- scale(dframe1$aspect, center = TRUE, scale = FALSE)

Currently, the model looks like this:

model1 <- glmer.nb(field_count ~ habitat +                    + sex+                    + agescale+                    #+ mon+                    + soil+                    + slopescale+                    + aspectscale+                    + offset(log(origarea)) #take into account field size +                    +(1|animal),+                    control=glmerControl(optCtrl=list(maxfun=20000)),+                    data = dframe1)

There were 24 warnings (use warnings() to see them)

warnings()Warning messages:

1: In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,  ... :
  Model is nearly unidentifiable: very large eigenvalue
 - Rescale variables?;Model is nearly unidentifiable: large eigenvalue ratio
 - Rescale variables?
2: In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,  ... :
  Model failed to converge with max|grad| = 0.0134799 (tol = 0.001, component 1)
3: In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,  ... :
  Model failed to converge with max|grad| = 0.148644 (tol = 0.001, component 1)
4: In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,  ... :
  Model is nearly unidentifiable: large eigenvalue ratio
 - Rescale variables?

etc.

So the model still fails to converge despite rescaling and altering the
number of iterations. I had also received the following error in relation
to month (in the reduced dataset there are only *four *months), so Ive
had to exclude it for the time being. I am not sure why I am getting this
error since the factor has four levels.

Error in `contrasts<-`(`*tmp*`, value = contr.funs[1 + isOF[nn]]) :
contrasts can be applied only to factors with 2 or more levels

I do eventually want to include interaction terms as previous analysis on
ranging behaviour suggests there is an interaction between age and sex.

Summary of dataset attached.  Also attached is the .csv file containing
the reduced dataset.

I have read various suggestions online and have come across the following
worrying line "It's perfectly possible that your data is insufficient to
support the complexity of the model or the model is incorrectly constructed
for the design of the study".

I would greatly appreciate any help you could give me with understanding
and solving the problems I am encountering with my model.

Kind regards,

--
Aoibheann Gaughran

Behavioural and Evolutionary Ecology Research Group
Zoology Building
School of Natural Sciences
Trinity College Dublin
Dublin 2
Ireland
Phone: +353 (86) 3812615