If you want all of the citations to be "Inger et al.", you will need to set uniquename=false.
This will not solve the sorting problem since still for biblatex all three articles were written by a different set of authors, and the bibliography is sorted first and foremost by authors. Because Mr Bearhop is second author in IngerEtAl2006b that paper comes before the two others which have Mr Ruxton as second author when sorting.
You can now do two things.
You could make your bibliography display only a maximum of two names as well, as in
\usepackage[style=authoryear-comp, maxnames=2, uniquelist=false]{biblatex}
Of course then all citations are sorted under "Inger, Richard et al." and then the title is consulted to sort the entries.
Alternatively, you can force biblatex to sort all entries under "Inger, Richard et al." by adding
sortname = {Inger, Richard and others},
to the entries.
In any case, now the citations will all be sorted as if "Inger, Richard et al." was the author.
Now the order still is not right since the alphabetic order of the titles does not coincide with your order, so we can add the sortyear field as follows (this is how it would look like if you also add sortname, see the MWEs below for more)
@article{IngerEtAl2006a,
title={Using daily ration models and stable isotope analysis to predict biomass depletion by herbivores},
author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Mackie, Kerry and Robinson, James A and Bearhop, Stuart},
journal={Journal of Applied Ecology},
volume={43},
number={5},
pages={1022--1030},
year={2006},
publisher={Wiley Online Library},
sortyear = {2006-1},
sortname = {Inger, Richard and others},
}
@article{IngerEtAl2006b,
title={Prey choice affects the trade-off balance between predation and starvation in an avian herbivore},
author={Inger, Richard and Bearhop, Stuart and Robinson, James A and Ruxton, Graeme},
journal={Animal Behaviour},
volume={71},
number={7},
pages={1335--1341},
year={2006},
sortyear = {2006-2},
sortname = {Inger, Richard and others},
publisher={Elsevier}
}
@article{IngerEtAl2006c,
title={Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis},
author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Robinson, James A and Jackson, Andrew L and Bearhop, Stuart},
journal={Journal of Animal Ecology},
volume={75},
number={5},
pages={1190--1200},
sortyear = {2006-3},
sortname = {Inger, Richard and others},
year={2006},
publisher={Wiley Online Library}
}
Where the a entry gets sortyear = {2006-1}, the b entry sortyear = {2006-2} and c sortyear = {2006-3}.
For another example of the sortyear field have a look at the knuth:ct entries in biblatex-examples.bib.
MWE for sortname
\documentclass{article}
\usepackage[style=authoryear-comp,
maxcitenames=2, maxbibnames=999,
uniquelist=false, uniquename=false]{biblatex}
\addbibresource{\jobname.bib}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@article{IngerEtAl2006a,
title={Using daily ration models and stable isotope analysis to predict biomass depletion by herbivores},
author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Mackie, Kerry and Robinson, James A and Bearhop, Stuart},
journal={Journal of Applied Ecology},
volume={43},
number={5},
pages={1022--1030},
year={2006},
publisher={Wiley Online Library},
sortyear = {2006-1},
sortname = {Inger, Richard and others},
}
@article{IngerEtAl2006b,
title={Prey choice affects the trade-off balance between predation and starvation in an avian herbivore},
author={Inger, Richard and Bearhop, Stuart and Robinson, James A and Ruxton, Graeme},
journal={Animal Behaviour},
volume={71},
number={7},
pages={1335--1341},
year={2006},
sortyear = {2006-2},
sortname = {Inger, Richard and others},
publisher={Elsevier}
}
@article{IngerEtAl2006c,
title={Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis},
author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Robinson, James A and Jackson, Andrew L and Bearhop, Stuart},
journal={Journal of Animal Ecology},
volume={75},
number={5},
pages={1190--1200},
sortyear = {2006-3},
sortname = {Inger, Richard and others},
year={2006},
publisher={Wiley Online Library}
}
\end{filecontents*}
\begin{document}
One \cite{IngerEtAl2006a}, Two \cite{IngerEtAl2006b}, Three \cite{IngerEtAl2006c}.
\printbibliography
\end{document}

Note how the sorting by names is severely messed up here.
MWE for maxnames=2
\documentclass{article}
\usepackage[style=authoryear-comp,
maxnames=2,
uniquelist=false, uniquename=false]{biblatex}
\addbibresource{\jobname.bib}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@article{IngerEtAl2006a,
title={Using daily ration models and stable isotope analysis to predict biomass depletion by herbivores},
author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Mackie, Kerry and Robinson, James A and Bearhop, Stuart},
journal={Journal of Applied Ecology},
volume={43},
number={5},
pages={1022--1030},
year={2006},
publisher={Wiley Online Library},
sortyear = {2006-1},
}
@article{IngerEtAl2006b,
title={Prey choice affects the trade-off balance between predation and starvation in an avian herbivore},
author={Inger, Richard and Bearhop, Stuart and Robinson, James A and Ruxton, Graeme},
journal={Animal Behaviour},
volume={71},
number={7},
pages={1335--1341},
year={2006},
sortyear = {2006-2},
publisher={Elsevier}
}
@article{IngerEtAl2006c,
title={Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis},
author={Inger, Richard and Ruxton, Graeme D and Newton, Jason and Colhoun, Kendrew and Robinson, James A and Jackson, Andrew L and Bearhop, Stuart},
journal={Journal of Animal Ecology},
volume={75},
number={5},
pages={1190--1200},
sortyear = {2006-3},
year={2006},
publisher={Wiley Online Library}
}
\end{filecontents*}
\begin{document}
One \cite{IngerEtAl2006a}, Two \cite{IngerEtAl2006b}, Three \cite{IngerEtAl2006c}.
\printbibliography
\end{document}

Note how the sorting makes slightly more sense here, but of course at the expense of dropping all co-authors.
uniquelist=falsehelps? - It seems to do what you want though the sorting might not be entirely to your liking.IngerEtAl2006agot a "b" andIngerEtAl2006ban "a" here, but that can be adjusted viasortyearif you insist. – moewe Jun 20 '15 at 19:00sortyear"? I triedsorting=nytbut this didn't solve problem – luciano Jun 20 '15 at 19:15biblatexdoes not want to sort the entries as belonging to the same author in the bibliography, unless you also specifysortname = {Inger, Richard}for all entries. You would then also addsortyear = {2006-1}for the a entry,sortyear = {2006-2}for b and finallysortyear = {2006-3}for c all in your.bibfile to get the desired sorting. – moewe Jun 20 '15 at 19:35